The way they work out that average can be arbitrary, because they have to convert non-numerical scores to numerical ones:
http://http://www.gamerevolution.com/manifesto/whats-wrong-with-metacritic-2791
Rottentomatoes has this problem too--you can find reviews that are debatable whether they should be labeled "fresh" or "rotten." That's why it's best to take aggregate scores with a grain of salt, and instead rely on a few reviewers with which you share similar tastes and that you trust.
As for the game, it's great that it's coming, but Im just not willing to shell out $40 for a game that's $5 during every steam sale, and is not guaranteed to have the DLC available for it at some point. I'd be more interested in bioshock infinite released in a much more timely fashion.