The worst dining experience we ever had at WDW was personal, and had nothing to do with the wait staff or the food. My husband and I almost got divorced in the middle of Via Napoli in September 2011. We scared the waiter and the tables around us, it was a nasty, NASTY argument. We were leaned in close to each others faces saying some very, very, very not nice things to one another. Our daughter was absolutely horrified by our behavior. Luckily, the trip got better after that or I might not still be married today.
Along the same token, (my husband that is), we had a very odd and tension filled experience at Sci Fi in March 2014. I don't really remember the exact conversation, it was the usual playful banter from the waiter, but I think my husband cracked a joke about how he was a lot better looking than me, which caused the waiter to rip my plastic menu from my hands and crack my husband in the back of the head with it, hard. As in, it made a loud enough noise that half of the restaurant turned and stared. I'm sure the waiter thought he was being funny, but our daughter and I were absolutely positive that my husband was going to jump out of his car seat and beat this man to a bloody pulp. (My husband has been on the ground in Afghanistan three times, and suffers from a touch of PTSD. The waiter had no way of knowing this, of course, but I don't think its ever a good idea to physically accost a customer, even if you think its all in fun. To his credit, my husband handled it in the best way he could. He didn't say anything at the table, but later into the meal he did slip away to the restroom and pull the waiter aside to have some choice words with him, away from the rest of the patrons.
In terms of bad experiences overall, Chef Mickey's ranks up as one of my two worst. Same complaints as the other posters here, long wait to be seated, filthy buffet area, kids running around like banshees with character handlers not doing their job, bland food. For some reason I was totally appalled that rather than Mickey they have you take a picture with that Mickey statue and charge you that ridiculous price for the photo. Don't know why that bothered me so much, but it did. Mickey never even got over to our side of the restaurant while we were there. My daughter, 7 at the time, hated it too.
We also had a horrible experience at Rose and Crown. Bad table, (right next to the fake hearth), bad service, bad food, all made worse by the fact that several members of the table behind us were three sheets to the wind, loud, obnoxious, and not paying attention to the toddlers and screaming baby at our table.
Be Our Guest dinner was also a bad experience. The tables are so close together you can pretty much share food with the people next to you. The people directly to our right had a pre-schoolish/toddler age little girl who had clearly been pushed too far, and proceeded to scream, cry, and throw food the entire meal. When our food came, I was shocked at the poor quality of my steak, and my sides were cold. We were so over it at that point, it wasn't worth it to send the food back, we just ate a few bites and asked for the check.
Chefs de France - horrible service, food was cold and when brought to the waiter's attention he took it to the kitchen where the plate must have been put in the broiler and the shrimp and scallops were burned and crusty....no apology...this was a $32 dinner....will not return.
