This is a really well-written and comprehensive article. It reminds me quite a bit of the questions the YC application pushes you to think through. I especially like the last point about competitors. What he outlines about finding others and learning from them is invaluable. There is not better, cheaper test than to analyze what already exists.
I was reading the beginning and thinking "Gosh, I could really use a local business search," and it's true - I've had a terrible time with Google and Google Maps trying to do that exact thing. I want businesses of a certain type that I can walk or take the train to.