Roger Ebert has a nice piece (now sadly offline) about the backlash against Inception. He laments,
In the “open marketplace of ideas,” it is believed, the better ones will eventually rise to the top. Sites like Rotten Tomatoes are where critics bring their ideas to market, but some readers come only to window-shop. It is a melancholy fact that for some, ideas have been replaced by the Meter reading itself.
I feel the same way about reviews that have to be summarized in stars, letter grades, percentiles, computer mice, thumbs-up, or whatever. To say that a television episode was a “B+” is often not helpful, whereas the full text of the review is much more informative. If I were a professional critic, I’d hate to have someone draw conclusions about what you’ve written after only reading that you gave something “3 stars.”
On the other hand, when I’m just scanning, I guess those grades are useful; when I’m looking for a new video game I find that I only read the reviews for “good” games – say, higher than 8.5 out of 10. That’s a shame, since, as Ratatouille noted, often the most fun reviews to write (and to read) are often ones that eviscerate the subject.