Of course expectations play a part...if you expect one thing and get something else, you probably wont be as open too it. Thats pretty normal.
However, I do not think this could cause a bad game to be reviewed as a good game or vise versa. I think it may have a small tug on the score in one direction or the other.
I think the bigger problem is the fact that most reviewers seem to consider 7.5 average which breaks the entire review scale.
EDIT: Well actually pretty much the whole review system is broken because of the environment these people are forced to play games in. Being forced to play through an entire game in a short period of time, without letting your opinions sit and without waiting to be in the mood to play the game definitely doesn't help the accuracy of the reviews and essentially says how much a reviewer would enjoy the game, not how much a consumer would enjoy the game...(this goes along with the point that most hard games get points deducted)
