Picking which idea to work on is the most important decision a founding team can make. Execution is important, but is too amorphous a concept to be called a decision by itself.
Team is important too. But while investors can choose between different founding teams based on their experience and such, the team is a constant once the founders establish it.
Four Guidelines:
1) Your idea needs to do at least one of three things:
Make something difficult easy, make something expensive cheap and make something that entertains
2) Pick something with a big market pain.
Market size isn’t always a great indicator of potential for success.
3) Commerce ideas are very different from social ideas.
Social software (e.g. foursquare, Instagram, Twitter) follows a different path than ecommerce. Compare this with ideas that have a clear business model from the start.
4) Pick something where you can empathize with your users.
Ideally, you would be building something for yourself.