I'll just give a quick rundown of the features, early days (well hours in fact,it was only about 6 hours ago) so the details are still coming out- feel free to add to the thread.
Full Flash support.
2 - 5 times faster CPU performance.
2 - 3 times faster web browsing.
Apps saved to SD card.
Use your phone as a WiFi hotspot.
Updated Android market, update all apps with one click or auto update.
Apps can utilize Android Cloud to Device Messaging to enable mobile alert, send to phone, and two-way push sync functionality
Microsoft Exchange support.
Older devices should be ok to run 2.2 as well ( Information still trickling out)
Should be out within 2 weeks- probably much sooner! ( Watch this very forum! )