Here are a few of my favorite things;
cut/paste
landscape keyboard
bluetooth peer-to-peer
in-app purchasing (think renewing subscriptions)
stereo bluetooth (A2DP)
search across all (or almost all) data in the phone
All in all, cool stuff! Developers can get their hands on 3.0 right now (or as soon as the dev site comes back!). To be released in “this summer”. Free to iPhone users, $9.95 for iPod touch (boo!!).
Coders Like Us 
Having played with 3.0 a bit, I have to say cut/paste works really well! A2DP works as expected and it is very cool to finally have that in an iPod! The little I’ve played with search has shown it to work well, but I don’t have a ton of things to search.