> You don't want to start a startup to do something that everyone agrees is a good idea, or there will already be other companies doing it
Which would show you that there is a market with customers. Now the question is, can you find a niche in that market and do you have enough to separate yourself from the competitors, also is it possible to attract attention to your product or is the market already saturated. No need to invent something totally new, it's fine to improve enough to make the pain to switch to your product worthwhile.
Unless it's a schlep: http://www.paulgraham.com/schlep.html