Just saw a bunch of you share your rating systems, so I thought I'd share mine, too.
I tend to rate books on how much I enjoyed/got into the reading experience. So I don't rate based on how well a book was written, though if the writing was terrible, that would likely affect my enjoyment.
No half-stars. I got so used to whole-star ratings that I fully commit. Sometimes I round up, sometimes I round down.
I don't think I've ever rated a book 1 star. If a book is really that bad, I likely won't finish.
I don't give out 2 stars often, but they're usually for really problematic books that are more annoying than entertaining.
3 stars is pretty much the norm, and I rate above or below accordingly when necessary. But 3 stars is the baseline for me.
4 stars are more common than 5 or 2. Enjoyment/involvement factor pretty high, but not flawless.
5 stars: I so wish I had more of them. I don't have enough of them. Maybe I'm just too jaded or cynical, or I just don't have enough love to give.