As I mull over the partly written novel that died on me last month, trying to figure out where and how it went wrong, it seems to me that the major problem was motivation. Not mine -- my characters'. I knew what kind of people they were, and I still like the basic set-up for the novel (which will probably turn up in some other piece eventually), but I think that their reasons for doing what I had them doing were just not convincing enough. And if their motivations didn't convince me, they sure as hell weren't going to convince you.
Thinking about this even more, it seems to me that people in "real life" are also largely defined by motivation. What a person wants plus what he or she is willing to do to get it -- that's a pretty good character-description-in-a-capsule. I'm not saying that one can always get what's wanted -- that's certainly not true. But what you want, and your means of getting it, define you.
Even if you're fictional. So now I'm thinking about the novella I'm currently writing. I have a completed, 27,000-word first draft. The story veers in the dangerous direction of combining science and mysticism, a sea on which many stories drown. This one may. But not on character motivation; this time I'm convinced of what my characters are doing.
That's a start, anyway.