Let me say right off the bat I hate people who give 10 or 100 scores. It means the game cures cancer, ok? Because if you give that game a 10, then a game comes out that cures cancer, you're saying Halo 3's gameplay will make you forget about cancer.
I also try to read reviews, but to me it's mainly the score that seems to stick. Which is unfortunate, since games' scoring system is so solidly stuck. The entire numerical range from 0-69 means "really bad"
I've also hated review sites that never re-review games after updates. Hypothetical example:
"Counter-Strike Source is a great game. But it's completely let down by its meager 5 maps, single character model per team, and multitude of bugs. 70."
Great. The game now has 15 maps, 8 total character models, and most bugs complained about were fixed.
Also, I kind of feel far too many reviewers prepare their review in their head before the game actually comes out. I like how IGN has a "longetivity" score, showing they've actually spent some time with it; but most of the time a 3-hour game that looks fantastic and has little gameplay will get higher scores than it deserves.