I've seen a number of prequels in the past little while, most from television - but I think most of the same factors apply to books, as well. (Except maybe for the fact that their recent popularity may be enhanced by the ease with which an actor can be made to look younger, with little touchups of easy CGI if necessary - which, of course, a book doesn't have to worry about. See "Surrogates," which - whatever you think of the storyline, and of course it's not a prequel - has some of the most amazing-yet-subtle CGI in movie-making.)
On the face of it, a prequel is a great concept. Any good story, in my opinion, has a sense that it is neither the beginning nor the end of the characters' lives: the things happened to them before the curtain lifted and things will continue to happen after it falls. There's a fine line between a story that lives in this way and a story that doesn't feel like it starts at the right spot ... but to me, I would rather trend towards too much backstory than too little.
Then, of course, there's the unintentional prequel - when you're forced to read book two and then come back to book one. This happens a lot with mysteries: those long-running series, good luck finding all the books, in order, in print. I've pretty much resigned myself to picking up whatever number I can find, and I hate - hate hate hate - reading things out of order. And because the mystery is so important, the mystery novel has an obligation not to spoil previous volumes, while still providing the reader with enough context to operate. Anne Perry's "Callander Square" does a marvelous job of discussing the events in "The Cater Street Hangman" without revealing a key aspect of the murderer in the previous book.
In fact, just to confuse the prequel issue further, I like to read things in chronological order, so I'll tend to start with the prequel ... but we digress.
I think prequels suffer from some potential pitfalls above and beyond regular volumes, and there are two that jump out at me:
1. That's All There Is? The events of the prequel seem too small / cramped to live up to all the foreshadowing and complexity of the hints in the main storyline. This is always going to be a big risk, because the reader (/ viewer's) imagination will probably create scenarios more interesting than what the writer can provide. Often, what we don't see is more powerful than what we do. But more generally, this can apply to situations that don't havethe right proportion of time or immensity. It's like finding out the mysterious vendetta that's been keeping our hero and villain at each other's throats for years is a parking space.
The "White Collar" prequel episode fell into this category for me. I felt that the now-time association between the characters suggested a far longer and deeper connection than could be summarized / presented in a one-hour episode. Even now, I sort of ignore the whole backstory episode and pretend there's more to it.
2. Too Tidy. The prequel takes in / explains every single little event in the present storyline. Sometimes the events even torque unnaturally to make sure that something is covered. Additionally or instead of, there is almost no content in the prequel that doesn't pertain to or lead into the main storyline. Life doesn't work like this. It's too neat, too contrived - even in storytelling. I stop believing in it ... and it loses one of the prime virtues of backstory, to flesh out the world with the unseen but present.
Not a prequel / main story situation per se, but this was part of my problem with "Abraham Lincoln, Vampire Hunter:" the whole alternate history was so entirely fixed / predicated on vampires that I had trouble buying into it. Sure, that's your subject matter, but the entire span of a person's life is more varied than that.
These aren't insurmountable problems, but they're definitely a significant concern and an impact upon (my) enjoyment of a prequel. What do you think, folks? Is there another pitfall I've missed?