Maybe they should have ended it at season 2?
I liked the first two seasons, one of the reasons is that it's different. Yea, there are many episodes that will piss you off, because of people just failing. That's human nature. While it's repetitive, the endings aren't always predictable. I'm often hoping that the person doesn't go through with the revenge, and when they don't it really stands out.
Started the third season, but dropped it for awhile like most of my anime. Like already mentioned, I prefer the music from the first two seasons. Not saying it's bad so far, will probably go back since reading this thread. But still don't think they really should have made the third season.
btw, Mushishi is awesome. Darker than Black was so so. And a favorite, Kino's Journey, is somewhat similar by showing different culture's behavior, yet not harshly judging them. Oh, maybe that's why I like Jigoku Shoujo. It's not too preachy about morals, and just showing what people generally do whether you agree or not.