Over on BookEnds LLC blog, there's a recent post about how to decode an agent rejection.
Let's face it, folks, you don't. Rejectomancy is for losers.
Here's my reply -
For your own sake, do not engage in rejectomancy. "No" means "no", and so does every other rejection letter.
Look at it like dating - you ask someone out and they say no, are they really obligated to give you a list of your undesireable characteristics? Would you really want them to? Can you believe what they say? And if you did anything about it, would it mean anything at all to the next person you ask out?
This person might say "too serious", while the next might say "no sense of humour". It happens. And there's no reason that it shouldn't, because we all have different ideals for a partner or a book.
If you are looking for a critique or a review, then go to the proper venue for one. There are lots of crit groups out there. (For instance, check out critters.org for speculative fiction.)
There are also lots of pros who will do coverage on your book or screenplay for a small fee. Make sure to pick someone with industry credits and who's not on the Editors and Predators lists as a scammer. Also, figure out what kind of feedback you are looking for before you pick a reviewer. The closer you get to your actual target audience, the better.
Once you have a work that promises something in the first 1/6 of the work and delivers exactly that by the last page, then you can look at how to market it to agents. And even if it's destined to be a best-seller, it's still a "not quite for me" for 95% of the agents out there.
That's because it really isn't for them. And if it isn't for them, if they don't have that spark that makes them truly believe, then they couldn't make it a best seller. For that, you need the one.
Do the work. Keep the faith. Keep submitting until it sells.