I'm a librarian who loves anime, manga, and reading a wide variety of genres.
I figured I'd jump on this bandwagon too.
My rating system is a combination of my emotions/gut feelings and letter grades, translated into stars. So:
5 stars = A
4.5 stars = A-/B+
4 stars = B
3.5 stars = B-/C+
3 stars = C
2.5 stars = C-/D+
2 stars = D
1.5 stars = D-
1 star = F
.5 stars = This was not ready to be published and, if I spent any money on it, I'm probably extremely upset about that.
All of this is based more on feelings than anything, and I sometimes bump things up or down depending on my emotions after reading something. I don't see 3 stars as being a bad rating. Granted, I'd love it if everything I read was worth more stars, but 3 stars means I didn't hate it.
Any ratings higher than 3 stars are for works I have at least somewhat positive feelings about, while anything lower than 3 stars is for works I have at least somewhat negative feelings about. I give out very few 5-star ratings and, for some reason, tend to be more comfortable giving out higher ratings to graphic novels and manga than to books (maybe because the artwork works on my emotions more?). I sometimes feel like I give out more lower ratings than higher ones, although my LibraryThing graph (pictured below) tells me this is not the case.
What it comes down to is that I don't really have an objective system. Also, I really wish I fretted less over assigning higher star ratings.