Transmog and Guides, that is all.
I actually really dislike the new comment system, I think it was easily the worst new feature.
Wowhead has been an awesome site for helping me through tightspots of world of warcraft. The guides themselves are the best part. However, a close second would have to be the site acheivements (not that I've gotten too many but) they make me feel like I'm still playing a part of WoW.
This is my goto place for guides whenever some new event pops up. Did the full "what a long and stange..." achievment last year with help from this site. I also really enjoyed the new content guides like firelands and such. Hence my vote went for guides.
New comment system is garbage and doesn't even let me vote correctly in some browsers.
I voted for Transmogrification sets and Site Achievements, with the Transmog sets being my favourite. Absolutely brilliant feature, thank you for that!
Wonder why no one add this to a list of top shield to obtain. http://www.wowhead.com/item=32082#.It's pretty darn good.