Basically, for songs (in my iTunes) as well as albums, my rating system follows this logic:

1 star: Skip
(Simply annoying or doesn´t work musically)

2 star: OK
(Routine stuff)

3 star: Very good
(Something memorable)

4 star: Great
(Worth listening to closely)

5 star: A certified classic
(I´m blown away)

Albums follow this basic pattern. Zero= absolute trash. Half stars indicate some doubt.