Off the top of my head, some of the 2.3 improvements over 2.1:
- you can move apps to the SD card individually. This was added with 2.2, actually. Just don't move any apps that have homescreen widgets.
- the keyboard placement and cut/copy/paste controls are much, much better in 2.3. Of course the Eris has the trackball, so cursor placement has been ok in 1.5 and 2.1, but selecting text in all apps is now much, much better
- the Gmail app is much improved. You can get priority inbox, separate notifications for multiple gmail labels, and even get notified with each new message rather than with the first message that hits your inbox (or that label, actually.) You can have separate notification ringtones for regular inbox, priority inbox, or even a label in gmail.
- the browser is much faster and better.
- the memory use is more efficient
- 2.2 and 2.3 add Google Voice actions, so you can use vocal commands to do a lot, including text entry in any field, quick voice dialing, etc.
Those are the highlights for me, anyway.