On the basis of real life, there is nothing perfect, and thus no game is perfect. A 10 MEANS that the game is a masterpiece, not a perfect game.Because to me a 10 means it perfect. In school you could only score a 10 if you had everything right, even if you made one mistake you'd get a 10-, 9,9 or whatever. But only a 10 if everything was right. The fact is that Halo3 isnt perfect just as I think there wont every be a game that is perfect so it shouldnt get a 10 but a 9,9 if you think its the best thing.
Every game has ups and downs, some stages and situations are more on the boring side of things, other levels are average and some others are incredibly fun. But no game is uniformly conceived or evenly fun.