Reviews of Halo 3, in fact, reviews of all games, have a terrible problem: what do you set as the standard? Once you have a standard, how do you compare another game to it?
Unlike movies, which are hard to rate as it is, video games don't have a "standard" length (about two hours, in movies' case). They don't have to have a story to be good. If you ask me, two games could have no elements in common whatsoever and still both could be good games. (Look at Geometry Wars: no story, no fancy graphics, only one mode of gameplay, still excellent.)
The other problem of rating is whether your rating system should be subtractive or additive.
In a subtractive system, you start with a full score, then deduct points for every problem. Bad graphics? Take away a point. Story idiotic? Another point. Some minor glitch? Maybe the game only loses a tenth of a point. Game breaking glitch? Maybe half the points are lost. This approach rewards conservative games that do only what they can do well. In this system, Geometry Wars would be penalized if it introduced a solo campaign with a story, or a multiplayer mode. The story (for example) wouldn't be very good, and that would become a new flaw.
The additive system rewards going for everything you can stuff in there. The GTA series is clearly pursuing this route. Like Tycho mentioned in Penny Arcade, if you just give them a score for each thing in the game, you'll eventually get up to ten. This doesn't mean the game has no flaws, though.
It's hard to get a rating system along either of these lines that doesn't occasionally deceive people. I suspect that most reviewers have a middle path: they give higher scores to big games, then dock points according to the flaws. Personally, I have a much better system than any of these: I see how much Ron Perlman there is in the game.
Halo 3 Review
Ron Perlman plays an Admiral called "Lord Hood". Appears rarely, although you hear his voice every once in a while, and he's in the end credits.
Halo 3: 2.5 / 5 Ron Perlmans
Now playing: Bob Seger - Old Time Rock & Roll