Horrible business deal that should have never been agreed to. The fact it was says a lot about Obsidian's stability -- it also says a lot about Bethesda, but most people already knew they were assholes.
I know it's not too commonly known, but having Metacritic bonuses in game developing contracts has become a well-spread standard over the past few years. I think it's horrifying considering Metacritic gets to select the sites and then determine their "weight" by themselves, in a completely opaque method of grading.
But it is pretty standard. It's not a horrible contract, it's a standard, and Obsidian is not the first dev fucked over by it (nor do I think this is the first time they have been).
... it is the mean of all the reviews.
It's not. Metacritic decides what sites/publications are in, and then gives them a "weight" towards the average. This is all done by them, internally, with no way of anyone knowing the method or results.