I disagree. A good idea is (at least) half the challenge. I've seen too many projects and applications that were developed flawlessly and meticulously with a fantastic design and code. But when they implement a bad idea, the end result is worthless and everyone wasted their time and money.
Of course, an idea is not enough. And (as a professional SW Engineer) I understand that a good design and implementation is crucial and very difficult. But the idea is what drives everything. Without a good, fleshed-out idea, you are never going to have a good app.
If the idea is a solution to a real problem then you've got something golden. Too many apps have cool features but don't solve any problems or only solve them partially.