I share your skepticism of Tempting Ideas — “no brainers” — that seem easy to do and of obvious value to everyone. If the idea really has both qualities, it would probably exist already.
Conversely, then, I wonder if there’s a third category: “Nonobvious Ideas”. That is to say, an idea that is easy to start, but hasn’t really been tried before — because everyone else thinks it’s wrong or cooky. AirBnB is a good example. (I suppose Nonobvious Ideas are also Hard, since you have to work so much harder to convince everyone.)