I like the fact that you now choose a spec and get bonuses bases solely on that choice.
I also very much like the fact that I got a Water Elemental at lvl 10 (Mage Frost spec talent) and that my Water Elemental's waterbolt does ~2x more damage per hit than any lvl <15 mob has
(Minor balancing issue there, but hey!)
Haven't been able to look around much yet. All my 80's are in Dalaran and there doesn't seem to be a working portal out of there. I just get DC when I try it :/
The UI changes are nice though. I especially like how the level-ups are displayed, and the new talents you have access to are flashed across the screen when they become available. The talent and profession windows look nice, although I'm not sure if they *work* nice yet.
I'll reserve judgment on the talent trees until they at least look like they weren't just randomly thrown together by the UI designers.