What's your criteria for picking the next project to build?

Here are mine:

• A twist or improvement on competitors so I know how I think I'm going to market it

• An idea of how I'm going to reach my target audience given my budget

• A problem small enough that I can build it myself in a reasonable amount of time with my own resources

• A hypothesis about why the market might want it and what evidence I'm going to accept as proof I'm wrong

I think the last bit is the most important. You have to have a falsifiable hypothesis and be willing to admit your hypothesis is wrong given some evidence to the contrary. Too many people are completely convinced their solution is right and no amount of evidence to the contrary will make them change their mind.

It's not just a problem in entrepreneurship, but also in opinions about policy, politics, etc. The best mantra I've heard is "strong opinions, loosely held."