For me, I miss only one major thing:
BBM - This is by far the biggest thing I've missed. I've tried most major apps out there for Android and nothing comes close to BBM. What I, and my friends, miss the most from it is true instant push messages and always-on group chat. When we all had Blackberries, my friends and I heavily used our BBM group in chatting, coordinating whatever we did, sharing photos, etc.
--GTalk is okay, but not always "on" and group chats aren't always logged in either. We've tried Yahoo IM, Trillian, and ebuddy, but they're just not that good at groups and group chats.
--PingChat is what we currently use now, but it's not without its own problems. We use it because it's cross platform since a few of our BB friends haven't made the Android jump (yet) and one of our friends is also on iPhone. But the notifications and push messaging system are atrocious. Sometimes I won't even receive group chat messages, or I'll receive them hours and hours later in a clump of 30 messages. The devs could do so much with it and make it THE preliminary multiplatform chat program, yet it rarely receives updates and I haven't heard anything about what they plan to do with it.
I'm still waiting for something better.
Others:
-BerryBuzz I don't miss as much. Although I liked the colored lights, Android's notification bar is a great quick way to see what new message I've gotten.
-I do miss the BB physical keyboard (Tour), though Swype is great. Maybe the new Droid Pro's keyboard will be tempting.