My rating system:
---------------------
Under 2 stars - Awful, un-listenable
2 Stars - Very Poor but listenable
2.5 Stars - Poor
3 Stars - Generally boring but it's alright
3.5 Stars - Good stuff
4 Stars - Great/Brilliant stuff
4.5 Stars - Pretty much flawless (I rarely give out this)
5 Stars - Perfection (I rarely give out this)

Most albums I review/rate generally get a 3.5 or a 4, which usually means that I liked the majority of it, spare a few tracks (some 4 star albums I rate are very close to 4.5, however I can't use a star to distinguish them from other 4 star albums)