Why the hell does everyone say it deserves 10/10? Or even 9.8/10?

Unless you have finished the game yourself, you dont have the right to grade it.

I finished it yesterday, maybe 2-3 hours before Gametrailers and Gamespot released their reviews. I was assuming a pretty low score, or... not "low", but NEVER as high as 9.8. Maybe a 8/10, because they are very hard on the grades(So I thought until yesterday at least).

I would've given it a 8.5/10 because of the whole package. If it was singleplayer only, it would be alot lower, like most reviewers have mentioned, simply because it was too short, too simmelar and some levels that where too reppetetive.

9.8 asks for innovasion, creativity and perfecness, Halo3 got none of those. Its just following an already stable formular.