I'm asked this question all the time. Is it team? Is it the idea? Is it product? Is it market?
The answer is that it is all of them and most importantly it is the way they all come together in a single company. Why is this the right team to do this? Can they package their idea correctly for the market? Is the market ready for their product?
I saw this 15 second clip at the end of one of the startup stories videos and it does a good job of explaining this "the way they come together" notion.