No, it wasn't one of the Blizzard devs, but a rather enthusiastic Russian player, Хируко, who managed to unlock every single achievement in World of Warcraft (that's 2057 at the moment) and an extra 205 of the approximately 300 Feats of Strengths unlocked. Technically, this means he has nothing left to do (collect missing mounts, maybe?) until the next expansion, Warlords of Draenor, arrives.
Here's a shot from his Battle.net profile. Yup, that's all of the currently available 21,995 achievement points.
And here's a shot from WowProgress, where you can see the leaderboards. There are a few other players missing one or two achievements there.
This guy even managed to snag a lot of Realm Firsts. He was able to level up every single class to max level first on his server. How's that possible? He transferred to a low population realm with his guildmates, and while he played on multiple accounts simultaneously, his friends were busy killing everyone trying to get to level 90.
Well, that's not a nice thing to do. No surprise that Blizzard's removing Realm First achievements from Warlords of Draenor.
And while maxing out on achievements in WoW has been done before, he's the first to do it all over again with all of the additional Mists of Pandaria and endgame content.