Fred Wilson's 10 Golden Principles Of Successful Web Apps

1. Speed is always the most important feature.
2. Your service must be useful right away, not after an hour of set-up.
3. Your software should have a personality and voice.
4. Less is always more when you first launch.
5. Make your app programmable -- allow others to add on to it in some way.
6. Individuals should be able to personalize your app (both users and programmers).
7. Make sure everything in your application has a unique, comprehensible URL (the REST approach).
8. Build your app so that it's optimized to be found in terms of both social media and search.
9. Make it visually clean.
10. Be playful -- you can turn anything into a game.

via www.businessinsider.com