Business books like to talk about the "hedgehog" principle:
http://ilead.byuh.edu/node/133
This is what it means in practice. It helps if all of the sets are relatively large, and the intersection is quite narrowly focused. Usually when I've had "small" successes (projects beloved by a handful of users, but not enough to make any money), it's because I was following my passion but not considering how many other people cared about the problem. When I've had outright failures (like the graveyard of projects that I never completed), it was when I followed my passion for a project that large amounts of people would want, but didn't pay any attention to whether the problem was tractable with the resources I had available to me.
