But if kernel is not signed correctly (by secret RSA key only Motorola has), it will simply refuse to boot. Therefore, you can replace kernel with custom, but it will not boot. So customisation is limited to what is possible by changing system files except kernel - this is not complete custom ROMs.
Also rooting dependant on security flaw in current bootloader. Presumably Motorola will patch it in some point in the future, leaving you with choice - either not to upgrade your phone firmware (except limited homebrew upgrading doable without kernel update) or lose the root ability, maybe forever.
I myself have switched from Omnia i900 to Motorola Milestone (GSM version of Droid.
Multitouch is enabled on Droid via custom firmwares/tweaks. Same with live wallpapers.
GPS is head above Omnia's, receiver is more powerful, and maps look great on Droid high res screen. Also magnetometer(compass) helps - you can instantly see where you facing, without moving.
Nexus is not better. Keyboard is personal choice, but Nexus is less robust - for example, no Gorilla glass protective layer. Also Nexus AMOLED blows in sunlight. Droid has transflective backlight, its unreally good outdoors.
Android phones use capacitive screens, so you don't press on them, you just *touch* them. Pointless to "press harder" anyway, since they employ hard glass screen covers instead of WM's soft plastic. Also on Droid its ultra-hard self-healing Gorilla Glass - no need for screen protector. They don't react to stylus, but will react to lightest flicks in much more responsive way than WMs.
In comparison to Android, WM was:
1) sluggish, Mobile Shell or not
2) ugly. Sure, you can skin some parts of it, but still 5-years old gui here and there
Two things I miss about WM:
1) security. If somebody steals your Android phone, very easy to crack it open for all contacts/login credentials inside
2) seamless Outlook sync in WM. Its far from being as good, universal and quick in Android. Also Android forces you to use Google cloud - which makes aforementioned weak security even worse.
This is not SIM-dependant, but phone hardware & operator-dependant functionality.
Btw, even when Windows Mobile does not show that it drops the connection during voice calls, this does not means that data could be actually transmitted simultaneously with voice.
This is for you to decide. Personally I didn't like AMOLEDs I've seen on Samsungs - they are too bright, with minimum brightness too high for dim light conditions, colours are too saturated, making ie people on photos look like they all having a sunburn, and also for some reason colour gradients looked not as "soft" as they looked on good TFT screens.
Maybe N1 AMOLED is different, though I doubt it - they all use the same manufacturer for now.
And no sunlight legibility is a show stopper for me. I use my smartphone as GPS in car, and I hate it when you stop seeing anything when sun gets behind you.
If you need a reason, I can give you one - AMOLED screens become totally unreadable in sunlight.
Mainly because they don't reflect light at all, and when brightness becomes inadequate to overcome a sun, screen just turns visually black.
One of the reasons I went for Milestone instead of N1.
Impressions so far - fabulous device in so many respects.
Looks like its bye-bye Windows Mobile for me, at least until MS finally will stop lagging behind :)
The only one WM feature I will surely miss though is the seamless and troublefree USB sync... I absolutely refuse to upload all my private information to Google just for the sake of syncing. Not because I don't trust Google, but because online account can be hacked/passwords leaked, etc.