I almost always have a good experience with WDW resturants. But I can think of two exceptions to the rule, one at a table service restaurant, and the other at a counter service spot.
I have eaten at the Sci-Fi in the Studios twice, and it was bad both times. The first time the burgers were tough, (as in laying under a heat lamp for a long time) slightly warm, and not worth eating. A couple of years later I decided to give the restaurant a second chance. Our party consisted of four adults and four children. After very slow service, we finally got to order, but when the food came (after a second long wait), two members of our party (myself and my daughter-in-law), were not served at all. We waited and waited, and finally, after everyone else had finished and were ready to leave, our cold burgers and limp fries arrived. The third long wait came when we asked for the check. Poor service and poor food. A real bummer.
I'll not be giving that place a third chance.
The counter service bad experience was at the Pizzafari in the Animal Kingdom. I was enjoying a solo visit to WDW, and after waiting in a long line I ordred a pizza and a beer. When I got to the counter to pick up my meal, the beer keg blew, and there wasn't anyone around who knew how to tap a new keg. The CM kept telling me it would be just a few minutes. After about twenty minutes the keg was finally taped, and I got my beer (with plenty of foam). The pizza, meanwhile, had been sitting on the counter all this time.
Happy to be finally able to eat, I found a table and sat down, only to find that my cold pizza had not been sliced, and in order to eat it I would have to pick up the whole thing and attempt to grab a bite the best way I could. That might have been possible with regular silverware, but trying to manage that pizza with those purple plastic forks made it next to impossible.
Giving it up as not worth the effort, I tossed the "pizza" into the trash can, and drank my beer while standing in line for the 3-D show. As the line moved into the "lobby" area a CM told me I couldn't drink my beer there. Since the doors were not yet open for the show, I ignored him and finshed my beer just as the doors did open (about ten minutes later). I tossed the empty cup into a trash can near the door, smiled at the CM, and enjoyed the bug show.
Yet another poor place to eat in my opinion is at Downtown Disney at a place called The Earl of Sandwich. Most un-Disney place at WDW methinks. In general it's a mess.
Karnak