I have to admit that I picked up Fruits Basket on a lark. I figured that I would try an issue or two just to see if it was my sort of thing. Everything about the manga struck me as the sort that I really dislike, and despite all of that, it is compelling and an utterly fascinating read. I think you've hit it exactly. It's the depth of the characters, not just Tohru, but all of them, that really takes it up a notch.