I also made the jump from my beloved Omnia to the Droid (on its first day, no less!), and have been a WM user since my first Treo 700w many years ago. So here are my own lengthy comments.
Bottom line: Don't let it be a 'tough call' - just go for it. Trust me, despite the weaknesses I mention below, you'll never look back. This phone is everything we all wanted/expected the Omnia to be, but it just never seemed to deliver. In comparison with my flashed-and-tweaked-to-death Omnia running the newest WM 6.5.1 builds: The brightness and clarity of the screen is amazing, and the capacitive touchscreen is just as good as you'd see on an iPhone. Even the lightest possible touch results in ultra-smooth scrolling at any speed. Every part of the UI is designed for touch access (a stylus or fingernail won't even work on a capacitive screen), and there seems to be better look-and-feel consistency between apps.
Multitasking is smooth and really works. The built-in task manager popup makes it easy to switch from one app to another. If a process hangs, Android automatically asks if you want to kill it - there's no need for the constant rebooting we faced with WM. I'm often running a dozen or more apps concurrently, and have yet to have memory issues.
I have noticeably better phone reception in fringe areas, and both audio quality and volume level of the speakerphone and multimedia apps blow the Omnia away. I use Google Voice heavily, so love its tight integration with the dialer. The main lock screen and the call-level screen locker simply work as expected. The Droid has a dedicated proximity sensor to detect when the phone is held up to your ear, and it locks the screen and turns off the backlight until you hold the phone in front of you again. No need for add-on apps like AutoCallLocker, which is a noble attempt to overcome a WM shortcoming but never seems to work quite right due to the Omnia not having a proximity sensor.
The GPS is an an incredible improvement over the Omnia: It's an open assisted GPS (aGPS) and thus gets locks almost instantly. I'm sitting indoors right now during a torrential rainstorm, yet still have solid locks on nine satellites! I was lucky to get 4-5 on the Omnia, even outdoors on a nice day. I find much more "location awareness" in Android apps (eg, the camera geo-tags your photos by default; and of course the new Google Maps turn-by-turn navigation features). Many of the apps are also voice-enabled including Google search: The other day someone asked me the score of the Redskins game. It was way cool to touch one home-screen icon and say "What's the score of the Redskins game?" The browser instantly opened with the search results, and the current score, video highlights, etc. were one tap away in the very first entry. I find that I use voice input a lot, because it cuts down on typing and seems to do a very good job in recognition. Voice also works with Google Maps navigation. (Now I just wish it could be used to create new emails or Word docs!)
Bluetooth works very well and seems much stronger and less noisy than my Omnia. Both phones will pair with the integrated BT in my car, but I always noticed that when the Omnia was on my belt, I had a lot of noise in the handsfree audio (the car's BT receiver is in a compartment behind the driver's seat), whereas the Droid is crystal-clear. I regularly connect via WiFi at home, and it works like a champ. The only exception is that you can apparently only access the Market via the 3G connection -- but the flipside is that ActiveSync to my Exchange server works via WiFi on the Droid and would not on the Omnia.
Some people complain about the weight, but I find its heft a positive. The whole device just feels like a solid, quality piece and is comfortable in my hand. It has a regular micro-USB port for charging, and a conventional 1/8" headphone jack (no weird dongles needed!)
Although I didn't expect to like it, I'm finding real benefits to the Android Market app: You can instantly search, download, and install tons of apps, tools, and games -- both free, paid, and trials -- and even the paid apps are inexpensive compared to the WM world. Much like using Firefox add-ons, Android's Market keeps everything up to date by informing you whenever new versions appear. Then it's just a tap to update the version on your phone while maintaining all your app-specific settings. Even updates to Android itself are pushed over-the-air. However, unlike the iPhone, I don't need Steve Jobs' blessing to install any app I want, whether it's available through the Market or not. It's easy to download an .apk installer file (Android's equivalent to the WM .cab files) from any website and run it. The only downside is that working outside of the Market means you don't enjoy the automatic updating features. Apple makes a huge deal of having 100k apps, but so far I've been able to find most every 'real' app I need. And with Android growing so rapidly, I'm sure its app catalog will grow to match or exceed Apple's over time -- especially since developers don't face a painful approval process and an iron fist deciding which apps to allow or reject. In particular, there seems to be no ban on apps that "replicate core functionality" (keyboards, email apps, task managers, browsers, etc.).
OK, now on to your question re. things I don't like as well as WM. In no particular order:
There's less user-accessible configurability. Android has a single 'Settings' menu that's not nearly as comprehensive as the multitude of control panel widgets found in WM. For example, there are no native controls of any kind for themes, colors, wallpaper, etc. (Fortunately, there are several 3rd-party apps to handle these things, and some will even accept theme files designed for one of the others.) There's no native way to create/change shortcut icons, size, or text. In those areas that do offer parallels to WM, I generally find fewer choices and less flexibility in Android. What I really miss is anything comparable to the WM registry, which allowed endless tweaking of fine-grained and undocumented options even when running standard OEM ROMs. Android's design philosophy is probably much better for non-technical users, but most people who frequent this forum will probably find it restricting without root access.
To answer your question, root access is currently not possible on the Droid. I'm sure someone will figure out how to do this at some point, just as they have with other Android phones, but so far it hasn't been rooted. I expect that many of my configurability issues and other complaints can be eliminated once I can get root access -- in fact, I've already found a number of useful apps and tools that require root access to work.
Unlike WM, there is no option to install an app to your SD card. Although apps themselves can choose to maintain their user data on the SD card, they don't all offer this choice. Just as with WM, this is a big issue: Although the Droid offers more free space in RAM than the Omnia, it becomes a hard limit as to how many apps you can install. It's frustrating when you have that huge 16GB SD card but can't use it except for your own data!
Both phones have 5 mp camera with flash, but I've been disappointed with the Droid vs. the Omnia in this area. While both give good results in bright outdoor conditions, the Droid just doesn't do as well indoors. Even at full resolution, I find photos overly-grainy and noisy. I have yet to find a phone with a flash that's as good as a regular pocket camera, but the Droid seems worse than the Omnia in terms of being overly-bright and narrow: It washes out the faces so they look white and over-exposed, while adjacent areas are so dark that they almost disappear. The camera software itself is much clunkier and hard to use, without the nice overlay graphics and menus found in the Omnia. It also has less manual controls than the Omnia, and zooming can only be done via onscreen +/- soft buttons - you can't use the volume buttons as with the Omnia. Auto-focus doesn't work very well, and is a frequent topic on Android forums. Most say it's a software problem that will be fixed in a December update, so we'll have to see. On the other hand (there's always an other hand, isn't there?), capturing full-motion video with the Droid seems to work better than the Omnia with sharp images and smooth motion, although I haven't done much of this so far.
The physical keyboard is a love-it-or-hate-it deal. I'm in the camp that finds it too flat and hard to touch-type quickly, but still find myself quickly switching to the hard keyboard for anything more entering a word or two -- mostly because I personally find the soft keyboard to be so bad. The capacitive screen is so sensitive that even the slightest brush of your finger causes a mis-typed letter (same thing when trying to select a tiny link in the browser), and there are no features to calibrate the screen or adjust its sensitivity. I'm also frustrated from lack of a Dpad wnen using the soft keyboard, as we had in the Omnia - there's a Dpad on the hard keyboard, but it's only accessible when that keyboard is open and in use. Without the Dpad, positioning your cursor within a block of text or correcting errors is almost impossible with the soft keyboard.
I really miss WM's global support for cut & paste! Although the Droid does 'have' C&P, it's just horrible by comparison. Support for C&P varies by app, and I've even found places where text copied from one app isn't available for paste in another. But worst of all, it's a totally 'bass-ackwards' implementation that renders C&P almost useless. Here's why: In WM (and PC Windows), you first select the desired text, followed by a click-and-hold to choose "copy" from the context menu. In Android, it's the exact opposite: you select "Copy" from the menu first, and then try to select the desired text. But you better get it right the first time, because the instant you lift your finger it completes the copy operation! Unlike WM, you can't make repeated tries to get the start/end points of the selected text right where you want them before invoking the "copy" (which is done by hitting the hard 'menu' key and then choosing from a menu, vs. WM's more-intuitive use of the context menu). All of this assumes, of course, that a particular app provides C&P on its main menu; and that the target app correspondingly provides a 'paste' function at the place you need to insert the text. You'd think that the DPad would help, but in many places (like the browser) it only scrolls the page or moves between fields vs. moving the cursor itself. For example, I simply needed to copy a text-only download URL from a web forum. The Dpad couldn't help, so I had to try doing it with the blunt end of my fingertip. This is very imprecise, especially when coupled with the "get it right the first time" design. After about a dozen tries, I just wrote it off as being impossible. We all complained about the Omnia's resistive touchscreen, but I'm quickly learning that there were real advantages to being able to use a fingernail or stylus for precision tasks like selecting text or tiny URLs!
So far, I can't find a soft keyboard I like. Beyond the Android keyboard, there are only a couple 3rd-party alternatives such as "Better Keyboard" (which I didn't find to be enough "better" to make a big difference). I was a real TouchPal fan on my Omnia, largely because it offered a special editing keyboard mode that made selecting, cutting, and pasting text very simple and precise. Although Cootek is creating an Android port, their beta version doesn't have the editing keyboard or many of the other features that made it so powerful in WM. I sure hope they can find a way to replicate these in Android, especially since its native C&P functionality is so worthless!
I never thought I'd say this since I hated Pocket Outlook, but if you connect to Exchange for email you'll find good old POutlook to be much more usable than the native Android email client. Android 2.0 includes ActiveSync, but the implementation isn't nearly as complete as you find in WM and some key Exchange features are unsupported. So like many, I'm now using a 3rd-party email app called TouchDown that comes much closer to POutlook, but still falls short in some areas. In particular, I find TouchDown insanely slow given the speed and power of the Droid. Conversely, if you've been drinking the Google Kool-Aid and run your life with Gmail and the other Google Apps, you'll likely find the Droid an improvement over POutlook.
I never used it so don't care, but unlike the Omnia, the Droid has no FM radio.
In general, I find that I have to use the 'menu' hardkey too often. Android apps don't have as many context-sensitive popup menus, onscreen buttons, or support for gestures such as swiping that we enjoyed in WM 6.5 and above. Seems like everything takes at least two steps - one to hit the menu key, and the second to select an operation. Often, it even takes a third step because icon-based menus don't have enough room for everything... so it becomes 'menu', then a click on the 'more' icon, and then picking from a scrolling list (it also seems jarring to encounter three different UI metaphors in just one operation!)
As many have said, multitouch is supported by the Droid hardware and by Android 2.0, but is not currently implemented in key apps like the browser and soft keyboard. Instead, you have to rely on irritating +/- softbuttons vs. a smooth zoom gesture such as using Opera on the Omnia. Hopefully, multitouch will be added in a future Android update!
No Adobe Flash support in the browser. When you don't have it, it's amazing how many sites you find that rely on Flash so either look awful and/or are unusable without it. Again, word is that this will be coming in early 2010.
That's all I can think of right now. Very sorry for the length of this post, but wanted to add it so other WM users could know some of the differences before they jump in. Hope it's helpful info. But to conclude, I return to my first paragraph: I'm very happy I made the switch, and haven't regretted it at all. The Droid is a far better device than the Omnia, as is Android vs. WM. Yes, Android has a few rough edges, but all can be fixed through software changes. Even now, I strongly prefer my Droid to the Omnia, and I'm sure that new Android versions will eliminate most of my issues over time. WM has had many years to evolve while Android is still quite new. Yet Android is already very competitive with anything else out there -- including the iPhone -- and will become more so over time. I think it's the way of the future for mobile devices, and am glad to have made the switch earlier than later.