Recently a US agent (Janet Reid) and a US editor at a major house (Betsy Mitchell at Del Rey) posted some statistics on what they had rejected and why in the past (but recent) period (10 months for editor, less for agent.)
Both the stats and the reasons might be useful to people who want to know what to work on before they submit a proposal. Their posts are copyrighted (and since Betsy's my editor, there's no way I'm going to annoy *her* by snagging her stuff and bringing it here!) However, I can say that the agent is reporting on rejections of completed projects--in other words, the proposal had passed muster, but when she got the completed manuscript, it didn't. Her reasons range from "just not good enough"--was in the top ten percent but not the top one percent--to specific problems with the characters, plot, or quality of writing. The editor reported on submissions (probably agented, but that's not specified.) Of 133 rejections, 36 were for "not what we're looking for" and 18 for subgenres not doing well in the market now. That's 54/133 "not right for the market" (either Del Rey's specific market or the market in general) and 43 were "just not good enough." (Other in-book problems are also listed.)
Both agent and editor expand a bit on these categories, so it's worth reading their actual posts, even though it means leaving this venue:
Though I never advocate writing to the market, once you've written the book, it's important to know what current market conditions are. If the market is flooded with the exact sub-genre your book's in, or if that sub-genre's sales are off right now, you might consider holding off on submitting that one as you write something else...once you get that first sale, it's easier to get others and the book that would've been rejected one year may be gladly accepted three years down the line.