The overwhelming majority of my experiences at Disney have been positive. However, I've not been to a lot of the restaurants I see mentioned the most here (LTT, Chef Mickey's, O'hana).
The only two really bad dining experiences we've had that I consider the fault of Disney both happened a number of years ago, so are not relevant now when considering adrs, but I will share just for reading interest:
Worst experience: Cantina San Angel, Mexico pavillion. I was a vegetarian at the time, and back then they did not have a vegetarian menu as they do now. The waiter told me that they could prepare the enchiladas without chicken, just cheese, which I said was fine. Well, when the enchiladas came out, they had chicken in them, so I sent them back. The chef came out of the kitchen, pointed at me and started yelling at the top of his lungs! The waiter pushed him back into the kitchen, and then brought me my replacement meal, which was the same enchiladas as before, with most, but not all, of the chicken removed. It was very disturbing not only to us but to others in the restaurant who heard the chef yelling. Needless to say we have not been back.
Second worst was nothing compared to the above! DH was served a practically raw veal shank (or possibly lamb shank) at Citricos. It was as if they seared it off then forgot the braising part! He got really skeeved and did not want to order a replacement or eat anything else. The server did do his best to rectify the situation.