Personally, if I had to give scores to games, I would review them as in golf.
A game that is nothing special, but is worth considering for every gamer will get a 0 for par. Great, Excellent, and Superb games will be scored -1, -2, and -3 (or birdie, eagle, and double-eagle/albatross) respectively. The best possible score is a -4, but that's impossible to get because nobody can hit the ball 480 yards.
Games that are not worth considering for everybody, get positive scores. A game where fans of the genre or the franchise would enjoy it, but others should probably stay away will receive a +1 score. There's no "worst" score since you can always take more strokes to get the ball in the hole, so if you really strongly dislike a game, feel free to give it a 12 or something.
Personally, I don't like the idea of giving numerical scores, but if I had to I'd do the above, if for no other reason than to screw with Metacritic.