When someone reviews a game at the Guardian they have to give it either 1 Star, 2 Stars, 3 Stars, 4 Stars, or 5 Stars. If you told that paper's editor that there was such a thing as a score of 3.5 Stars, or 4.5 Stars his head would explode.
Then give it a 4 star if it really gives that much love.
Not to take any sides, but 3 out of 5 in the "good" in the 5 stars system, whereas when it's translated into percentage it's 60%, which is "mediocre to terrible". If this same reviewer used the percentage system, I bet he would score above 70 at least, probably in the 75-80 range.
Therfore, I don't think metacritic should accept scores that are given on a 10 point scale or lower. For example, Gamespot uses the 10 point scale (1,2,3,4, to 10,....no decimals), and if some thinks the game is deserving of, lets say, 8.5, the reviewer is forced to round to to an 8 or round up 9, higher or lower than what the reviewer actually thinks of the game.
IMO metacritic should take any reviews under the 20-point scale (1, 1.5, 2, 2.5, 3, to 10, counting by 0.5), and encourage review sites to use the 100 point scale (1.1, 1.2, 1.3, like IGN).