Given the mega servers and the ability to choose a subset instance of like minded player [snip]
....where did you hear that there are voluntary subsets to the mega-server? Are you talking like a choice between PVP, PVE, RP servers? Or are you talking about the ability to just have friends/guildies stay with you on the same instance that they've talked about before?
I'm hoping it's the earlier... because mixing RP players with the (for lack of a better term) Warcraft-minded players is going to end VERY badly...