I enjoy World Events in WoW. It changes the landscape a little and the world is a more active than usual. It causes some extra lag but I don't spend that much time in cities anyway.
What I really enjoy is the fact since I ran myself ragged last year, this time around I don't have to scramble to get a bunch of achievements done. I have my violet proto-drake and the few achievements I don't have aren't part of any other rewarded meta I know of, so I can say "meh" to doing them (Peddlefeet, Masks for All Occasions, Brewfest mount, etc.).
Of course Pilgrim's Bounty (Nov 8th-14th) was added later, so I have one last (?) event to spend extra time playing on.
I can't imagine they'll add too many more world events. If you're like me, they sometimes get overwhelming, for instance I don't think I did anything with Harvest Festival (Sept 27th - Oct 3rd )because it ran so close to Brewfest (Sept 20th - Oct 5th).
I like that they add a few new activities like BB King but I also like going into the event knowing I've done the bulk of it and I don't need to log on every day trying to get something done. And if I ever got the urge to do so I could just do it on an alt I've neglected. (That time won't come!)
3 days ago