I'd like to add to this -- in my opinion this goes in cycles, and it's important to ride out a couple of cycles. The cycles are ideating -- dreaming and researching how big a change the idea encompasses, and critiquing -- brutally throwing every criticism you can up against the idea.
So most of the time I'm not a huge believer in ideas -- it's execution that matters, and great teams can make money starting with mediocre ideas. But then every now and then one comes along that makes me pay attention.
So I go through the research, digging through competitors and looking at market-entry obstacles, partnership-leverage opportunities, and technical difficulties. If this is done correctly, at the end it's kind of like whittling away wood to make a whistle: At the end I have smaller piece of something that still looks cool and might work.
Then, if I want a whistle, I execute on that some, taking it to market or otherwise throwing as many rocks at it as I can. As I execute, it makes the idea more powerful -- only to go through another cycle of adjustment and pairing as the product meets the marketplace and you take another hard look at it.
In my opinion, after a couple of cycles you begin to either understand all the reasons why it won't work (and are executing anyway) or you understand all the awesomely cool things it might be (and have decided that it's not worth the effort).
Hope I explained myself. Tough concept to cover in a few words.