IMO the fundamental problem here is telling apart great ideas from no so great ones (if it were easy, the people with genuinely great ideas would easily secure

IMO the fundamental problem here is telling apart great ideas from no so great ones (if it were easy, the people with genuinely great ideas would easily secure funding). Unfortunately, it's generally not possible before the idea is realised - all great ideas seem very risky at the beginning, some even look downright silly. There are no safe horses to bet, and this affects both VCs and you as a developer willing to devote afternoons to someone else's idea.