Paul Graham writes about how to generate start up ideas. There are a lot of good points here. The well metaphor is kind of the flip side of Chris Dixon’s horizontal vs vertical discussion.
When a startup launches, there have to be at least some users who really need what they’re making—not just people who could see themselves using it one day, but who want it urgently. Usually this initial group of users is small, for the simple reason that if there were something that large numbers of people urgently needed and that could be built with the amount of effort a startup usually puts into a version one, it would probably already exist. Which means you have to compromise on one dimension: you can either build something a large number of people want a small amount, or something a small number of people want a large amount. Choose the latter. Not all ideas of that type are good startup ideas, but nearly all good startup ideas are of that type.
Imagine a graph whose x axis represents all the people who might want what you’re making and whose y axis represents how much they want it. If you invert the scale on the y axis, you can envision companies as holes. Google is an immense crater: hundreds of millions of people use it, and they need it a lot. A startup just starting out can’t expect to excavate that much volume. So you have two choices about the shape of hole you start with. You can either dig a hole that’s broad but shallow, or one that’s narrow and deep, like a well.