Here are some of the criteria I use in determining whether an idea is worth my time to develop:
1. Is the idea high-concept; i.e., easily pitchable?
No, this is not me happily internalizing the marketing-driven world of today's studio system. I just want to make sure that people will read what I write, and I'm not exactly a known quantity in Hollywood (despite having 136 followers on Twitter, at least half of whom are not spam-bots!). But if I can sum up the appeal of the script in a zippy logline, then a stranger in a position of power just might deign to pick it up. To quote one of my favorite movies, these are the facts of the case, and they are undisputed.
2. Can I actually pull it off?
If coming up with a script that fit criterion #1 above were all it took, then L.A. would be stuffed to the gills with millionaire scribes. Great high-concept ideas are a dime a dozen; people who can execute those ideas are rich. A humanoid dinosaur and a supermodel travel back in time to stop the assassination of Abraham Lincoln. I would write that if I thought I could pull it off, because why not? Someone's going to read that script if it comes across their desk. But I don't think I have the chops.
Managers pride themselves on knowing their clients' creative strengths and weaknesses. As my own manager, I have to know mine. There are certain types of characters, certain types of scenes, certain types of plots that I know I can do well. I'm good at snappy dialogue and unexpected injections of humor. I can write a passable action scene, but I'm better at building and sustaining tension. These are all major considerations in the kind of script I choose to write.
3. Does the idea of writing this excite me?
This is a no-brainer, and should be for anyone writing a spec. If you have the power to choose what you write, choose something that really sparks your interest and makes you look forward to sitting down at the computer. Even the scripts that most ignite your passions will cause you pain and frustration at some point (if not many points), so it's essential to find a project that makes all that suffering worthwhile.