If I could change one thing about myself, I would choose to have one driving passion (apart from my lady, who is the exception to the no-passion rule). All my life I've been a Jack of all trades (and master of a few) but I've never had that one consuming passion. It affects everything: projects are hard to finish, I have too many interests to concentrate fully on one or two, I'm constantly distracted by anything interesting and new things to learn.
The up-side is that I have a ridiculous amount of knowledge on all sorts of bizarre subjects (as well as useful ones) and that I never, ever tire of learning. But I'd still swap it for an all-consuming passion because then my work ethic would be a lot more solid and I could keep pushing and pushing to achieve something major instead of farting around with lots of disparate ideas at the same time!