Pretty much what Michael said. Arstechnica does a great job with reviews (buy, rent, etc). I think Kotaku does too. Joystiq only implemented a rating system last year because (presumably management) wanted to be included in Metacritic.
The other problem with reviews, that Al pointed out is the time necessary to really delve into it. The quality of reviews suffers a lot, as was the case with Mass Effect 3. A lot of reviewers did not finish the game, or perhaps they were paid to omit the ending backlash. But that's another debate.
Since I've only slightly expanded upon what others have said, I wanted to add that there is yet another problem with reviews. Upon reading many, I can tell the reviewer did not really enjoy the game. So we have reviewers either biased towards or against the game, which again affects the quality. And the end, they give a 4/5 (for example) after a scathing review! They're nuts! Or paid..
Happy thanksgiving to those who celebrate it!