I took a course with a professor who is an entrepreneur. Some tidbits of what he said:
If you have an idea stop right now. Find a problem.
Ask pointed questions, but if you get too specific you'll only cheat yourself. Example (to steal someone else's) is the dog nail clippers. If you ask "What troubling you these days?" they can say anything from the market to the weather. If you say "What's troubling you about dog maintenance?" you can probably end up with nail clippers. If you ask "What's troubling you with cutting your dog's nails?" you've shot yourself in the foot.
As far as criteria for startup goes it went something like this
1) Billion+ dollar market. 1% of a billion is still a million.
2) Growing at 5% a year. Even if you do nothing your business is growing.
3) Money is spent on it already. No one spends money on it? You're either a genius or no one cares. For instance, deodorant. Somehow some people convinced us all that we need to smell good, so we need deodorant. So while you could make the next deodorant odds are you're not. As a corollary to this there will be competition.
4) Control your own distribution. For example: Magazines are difficult to sell. It's $$$ upfront to even put your magazine on the rack in a store, and they don't guarantee sales. You don't want this.
5...) Find some criteria that you would be happy with on a personal level. It could be that you reject running a business a certain way ("It has to be moral in this way") or something like that. Don't limit yourself to one technology or revenue stream.
Now go through your list of problems and find something that matches all these criteria. Pick something that matches.
Are their multiple matches? Pick whichever you feel strongest about.
2 or more tied? Pick one at random. You made the criteria because anything that matched it would be something that you would be happy doing and would be a good business. So just pick something and go do it.
Disclaimer: Even if you do all this, you could still fail, but you've just reduced the odds compared to every guy with an idea looking for a problem.