In my case,
5 stars = Does exactly what I want;
4 stars = Does most of the things I want, minor features missing;
3 stars = Has a few useful features, the rest are irrelevant;
2 stars = Frequent crashes and major bugs;
1 star = Does not install or start at all no matter what I try (very rare).
I mostly give 4 stars, but there are a lot of 5 star worthy apps, as well (Pocket Informant and ES Explorer, for example).