Good question. I think there was a poll about this some time ago.
Before I say our do not return, first a disclaimer. My wife and I visit WDW frequently (~4 visits a year) and enjoy a more up-scale experience but we are also happy with pub food type of places. We value good service highly. We recognize that Disney restaurants seem to go through phases where one is up and an old favorite is on the down-slide. With so many places to eat at WDW it doesn't make sense to go to a place that we've had bad experiences at - even if they have been improving. So, the list?
Fultons - poor service, food just OK.
Big River Grille & Brew Works - Very poor service, food often not prepared properly
Mexico - OK service, OK food, overpriced
China - can get better right here at home
Italy - better food and service right here at home
Rain Forest Cafe (either one) - poor service, food just average, atmosphere way over-done
Planet Hollywood - very poor service, average food, overpriced