Here is a short list containing some of the major improvements according to AndroidDevelopers:
- 2-5X faster performance thanks to the Dalvik JIT compiler
- Cloud to device messaging
- Application backup
- Apps on SD cards
- Portable hotspot
- New Android Market features
- App error reports
Adobe Flash 10.1 is still planned on being supported in FroYo as far as I know.
There is also a video on Youtube introducing Android 2.2. Have a look now, because you will be waiting awhile since we have no firm release date for 2.2 yet.
Image from BoyGeniusReport
Source: Android Developers