I've been reading through the SDK notes to sort that out. Here is my personal read.
There are a bunch of features dealing with integrating additional bits into the contact list. HTC already did much of that with Sense, but that's an HTC special right now. Base android catches up a bit in 2.0 to the experience we've already got. I suspect access to these APIs will make sense's contact list even better.
Bluetooth got quite a number of new APIs. There is some interesting stuff in there for bluetooth proximity sensors, which should make for some interesting social apps.
The browser got a major update in 2.0, which should give a nice speedup.
The other thing that seemed interesting is if I'm reading it correctly there is now an actual Android API for multitouch events ("MotionEvent can now report simultaneous-touch information for devices that support it. Up to three pointers can be tracked simultaneously.") We know the Hero can do multitouch, as they put pinch and zoom into a number of apps, there by making this a nice win for Hero owners. Up until this point there wasn't a way for app writers to do multitouch, even if the hardware supported it.
A lot of the APIs are hardware support that doesn't do us any good. New camera things, the dock, etc. Not every part of any android upgrade will be applicable to older phones.
Overall, really looking forward to the upgrade. I already have such a device crush on my Hero, and think this will make it even better.