I've been mulling this over a lot lately. Miss Snark claimed at one point that all a synopsis had to do was be short, not painful to read, and show that the author hasn't screwed up somewhere in plotting the book. I'm not sure that I agree. Those seem like good minimum conditions, but I think I want more from my work than to demonstrate I haven't screwed up in the minimum number of words.
I want to leave the reader with questions that interest them enough to want to read the whole manuscript. This does not mean questions about what happened--those are by way of screwing up, because the reader of a synopsis needs to end their perusal knowing what happens. What I'm talking about are questions of method. I want my reader to say something like That's cool, I want to see that or, Really? Why didn't I see that coming, I have to read this, or just, oooh, nice.
A well written synopsis gives conflict, plot, setting, character sketches, and some genuine flavor of the book, at least in my opinion, and if that takes slightly longer, I think it's okay. I keep coming back to the idea of talking about what excites you about this story as a writer, because that's what's going to convey the important parts of the book's flavor. Perhaps this is another instance where strong voice is important.
So, what do y'all think? I'm still formulating my opinions on the subject.