The perspicacious Peter Suderman writes:
…the critical medium that suffers most [from overly positive reviews] is pop music criticism, which
skews toward generally positive reviews of most everything, no matter
how bland or terrible. Scan the sidebar of Metacritic’s music page.
Nearly all of the review averages are positive or very positive, and
almost none of them are straightforward pans. In fact, right now I
don’t see a single album with a review average that gets a score
categorized "generally negative reviews." Contrast this with the movies page,
which contains more than a dozen films with low averages. Even the
limited release indies – the "artsy" films – are often given low marks.
But why? When it comes to a movie, you might actually go see the movie if you read a good review. Therefore the newspaper must be careful not to mislead you too many times and that implies a certain amount of criticism. But even a well-reviewed CD you are unlikely to buy, if only because there are so many CDs out there and there are so many well-defined genre preferences. So the MSM source courts many good music reviews, to give readers a sense that they are learning about "interesting product"; in any case only the fans will buy the stuff.
One testable prediction of this hypothesis is the following: when musical taste was less fragmented, and a review was more likely to influence buying decisions, music reviews would have been more critical. Similarly, if the outlet is pure niche, and thus being read by potential buyers only, the reviews should be more critical as well.
In the comments on Suderman, William Brafford comes close to this view.
I might add that Washington Post restaurant reviews are far too positive. If WP readers were simply told "There are hardly any good restaurants in your crummy little city," this wouldn’t do much for WP circulation or advertising revenue.
The less that people buy books, the more positive book reviews should become.