The Artifact and pvp systems look interesting and fun, but I'm most looking forward to being able to swap between all specs for each class without bounds and the same going for swapping talents. The new challenge modes also look like a fun way outside of raiding to get to acquire gear, since it changes week to week, it'll at least keep things somewhat more fresh for a longer time than grinding raids for months on end.
As for a favorite moment in WoW, WotLK will probably still be one of my favorite expacs as far as raiding and group content went, as no raid since ulduar has given me nearly