That to me is a more interesting and logical question.
Often these great ideas are hard. PG has an essay on this (http://paulgraham.com/ambitious.html). Better search is a clear example. Google is amazing, but it isn't perfect. Yet being better than google is really, really hard.
I think one of the tricks of being a founder is to pick the greatest idea that you are capable of implementing, with the caveat being that picking something that you are capable of implementing right now is probably too conservative.
Ideas are very important.
1. Without an idea there is nothing to execute on.
2. Depending on how much you believe in the idea it is what will give you the stamina to continue no matter what obstacles you will meet. (This can some times be a problem, but often and advantage)
3. Without and idea there is nothing to validate against.
I don't know about you but in design I always found it a lot more effective to have an idea before i start designing instead of just make it up as I go.