pick large, difficult markets. it won't matter... a rising tide lifts all boats. conversely, if there is no one doing the exact same thing, if may be too narrow... a feature not a product... or maybe the market is not there yet.

I've always found that telling people about what I'm working on led to progress, activity. whereas... working in isolation led to more isolation.

also, don't spend to much time looking for direct competitors or similar ideas. this has, in the past, caused me to spin my wheels and never get anything built. or in one case, continually redefining it so it was unique. I was building a product for a slidedeck, for investors, and not for the customer. kinda pointless -- regrets.