Ok, here's a quick one. You have an idea for a business. Should you pursue it? Is there a market? A demand, in other words?
Well, is anyone else doing it?
If not, then you have to ask yourself whether there's really a demand. A truly innovative idea means that no one has thought of it yet. This is very different from "People think of it every day, but there's no demand for it." Because if people have already thought of it and chosen not to pursue it, you'll never know. It's a risk.
However, if people are doing it, then your challenge is completely different. Now you're stuck with, "Well, people are already doing it. Therefore rather than simply act on my idea, I have to compete for a piece of it." There are times when this is a fine approach. The world always needs more dry cleaners, gas stations and pizza shops. You can indeed do the same thing as the guy across town and simply do it better. But what if you truly thought that your idea was one of those ones from up there in the first category, something truly innovative? It's a definite mental shift (heck, it's a mental jump off a cliff) to go from "My unique idea will be something no one has ever seen before" to "Oh bugger, there are dozens of people already doing it, which small piece of the original idea can I spin so that I have a hook?"
Just one of those random thoughts that keeps me from making the leap. If I don't see anyone else pursuing my idea I can assume that there's no market. But if people are doing it, then I don't want to play because I'm interested in innovating, not in competing.