My favorite addition is that users' comments will now be annotated with the patch number that was in effect when the comment was made, so you can more easily spot information that might be outdated (see screenshot).
Alchemical recipes that are attained via discovery are now noted as such under the "source" tab, and the corresponded spells will state their provenance as well. Elixirs will now be annotated as to whether they are Battle or Guardian elixirs. Pet skills will now show which pets can learn the skill, as well as which mobs can teach the skill. And finally, tradeskill spells will show at what skill levels they change from orange to yellow, green, and gray.
I say well done, Wowhead folks! If you have any ideas for how Wowhead can improve, hit up the their Site Feeback forum.