I feel the next WoW x-pac will be another boring faction grindfest with some pop culture references thrown in. (The reason I left in the first place. Plus they totally ruined PvP, starting with BG's and crashing and burning the rest of the way with 'Arenas'.)
I feel if you want fun PvP try out Warhammer Online. I still feel instances are the social bane of MMOs though.
I play around with Mangos and it's really really good, is very open source, updated sometimes twice a day, and has gotten a lot better since TBC was released. They recently re-did all their world code and it runs much better. Plus, I like compiling my own code and contributing SQL to the project to help fill things out. Like I do here with quests when I can. It has a good community behind it as well.
Recently they have begun adding support for PostgreSQL (I am set up with 8.4 and am working to port my stuff over.) as a nice open source alternative to MySQL, which is wonderful.
WoWEmu is very bad, most everything does not work and :P I don't want to have to hack my client to get it going - which is a big no-no.