The biggest thing I've missed so far going from Android to the Lumia 900 is the Google Voice integration. I'm using GoVoice for texts, but its too much trouble to use it to dial. With Android, everyone's callerID showed my Google Voice number when I called, but now they see my ATT number. If anyone knows a way to address that without having to use the GoVoice dialer for all outbound calls, that would be great.
I also have a Google + app on the wishlist, but there's only a few people I interact with on there right now - the mobile site is good enough for now.