Edit: I liked CoD4, I just mean that if something's a problem, then reviewers should highlight it as a problem. They shouldn't forgive it just because they're hyped/like the developer.
What planet are you on?
Gaming magazines pretty much allways give high scores to hyped titles, aspecially if the publisher behind it is a big one.
There is a reason for why the average score of all games across all review sites is above 70% and not around 50%.
The reason is that gaming journalism relies on hype, nobody wants to read about some average game coming out each year, we want to read about some super amazing OMFG game. Now, since there are few titles that can actually be amazing OMFG games, gaming journalism has to hype stuff. They get previews, they hype stuff up.
End result is, that after hyping a game so much in the previews, the journalist would end up looking like a moron if he didn't give the actual game a decent score in the review. Thus imparity.
This, coupled with the fact that publishers often spend alot of money on gaming journalists, flying them to see new games in the works, sending them games and extra's etc, means that the gaming journalist has to watch his relations with the publisher. You wouldn't want to give Halo 3 a 5\10 (if it really was that bad) because microsoft has a $50 million marketing campaign behind it, and if you start to ruin that hype by a bad review, you can bet your ass that you will not be recieving games and invitations to see new games in the works again.
Basically: Trusting gaming journalists is like trusting a used car salesman.