Patch 3.0.2 brought Achievements to WoW, and so far they're proving to be a lot of fun. I've explored and finished a few low level quests in Eastern Kingdoms to unlock the exploring and questing achievements. I went on a raid to kill Hakkar and Ragnaros (getting people who could douse runes was harder than the bosses). I've done several of the Hollow's End achievements. I even joined a large raid that attempted to kill the Horde bosses for the Black Bear mount. Unfortunately, it turns out that 120 Alliance attacking Cairne Bloodhoof crashes the server, so that Achievement will have to be tabled for later.
Judging by the flurry of activity, many other people are enjoying Achievements. Blizzard has done a really good job with them, I think. There's a really nice mix of Achievements of various difficulties.
WAR has a similar mechanic, Tome of Knowledge Unlocks, and it's interesting to compare the two systems. There's one huge philosophical difference. WoW shows you all the Achievements available, while WAR hides them.
I find this completely changes how one approaches Achievements or Unlocks. In WoW, Achievements are something you work towards, while in WAR, I found that Unlocks happened as I was playing. I never sought out any Unlocks. If you are seeking Unlocks, you generally have to do random things because you never know what will trigger an Unlock.
Of course, I'm sure that eventually someone will come up with an Internet database listing all the Unlocks that people have found.
In a weird way, how the two games handle quests is the inverse of how they handle Achievements. WAR goes out of their way to provide you with directions for the quest. I've noted before that I really like the quest and map integration. But WAR provides you with zero indication about Tome Unlocks. You can guess at a few of the obvious ones, but others are not obvious at all.
You could say that WAR is more Explorer-oriented, but I'm not sure that's the case. In some ways, it's more of a "guess what hoops the designer wants you to jump through" rather than really exploring.
I found it really interesting how the two games have very similar systems, but one single design decision--hidden Unlocks vs known Achievements--completely changes how you approach the system.