I'm not going to select bad shows I've never seen much of (Barney, Teletubbies, Mighty Morphin etc.). I'm picking shows that at some point I watched before it hit me how bad they were.
American Idol - it never ends. Even after a season, you have to endure songs, commercials, and various other appearances by the winner (and runner-ups). I watched the first season and I watch the beginning of the next few seasons to see the bad auditions.
Jerry Springer (after they took the fights off) - I would watch this if I caught it on. I always liked the "I look GOOD" nasty chicks that would be on there. It just never was the same after they wouldn't show you people duking it out.
The Simple Life (nuff said)
Scooby Doo - I seem to be alone in this category, but I hate the original cartoon, the movie(s), just about anything to do with that "gang"