Diablo 3 doesn't have any critic reviews on Metacritic yet. I imagine when it does, Blizzards new game with get a score representative of what it deserves, because I can't help but feel that the 3.4 it currently has as a userscore isn't indicative of the quality of the game. Remember, at this point, the game has been out only a little over 24 hours – made more impressive by the fact that nearly 1200 people have taken the time to write a negative review.
Most complaints seem to be about error messages, mainly showing because the servers are being hammered (an issue that usually only lasts a couple of days and that Blizzard have experience in fixing from the early days of World of Warcraft). Others are complaining that the series has changed to much from previous games, although it's difficult to know whether those complaining have actually played the game.
Diablo 3 is now one of many titles – including Modern Warfare 3, Battlefield 3, Mass Effect 3 – which Metacritic users have tried to destroy via user reviews.