- Coolness - If a potential user encountered this app all alone, with just a few sentences to describe it, would they likely be excited about it?
- Virality - How likely would they tell their friends about it?
- Monetization - How much money does each user translate to? Sometimes the right crowd (people wanting to install awnings in their home) is better than a big crowd (people looking for free software).
- Stickiness - How likely would they use the app for long periods of time?
It's interesting to think about the top Facebook apps in these terms.
Top Friends - Slightly cool, very viral, not very monetizable, not very sticky.
Graffiti - Very cool, not so viral, not very monetizable, pretty sticky.
iLike - Cool, viral, monetizable, sticky.
... and so on