None of my visits to sit-down meals at WDW have been bad, but some have been disappointing:
1) The one in Morocco at Epcot. I was expecting foods that were a little more exotic, out of the ordinary. What I got sounded awesome, but mostly ended up tasting bland, and seemed to be very Americanized.
2) Garden Grille in the Land at Epcot. Nothing special. I can get everything on that menu at home at it will be tons better.
3) Princess Breakfast at Akershus in Norway at Epcot. Really boring breakfast, extremely limited choices. We have only done this once, so my DD could see the princesses.
4) Mama Melroses in the Studios. Again, nothing special. Was hoping for something more than your typical franchise Italian.
5) Rainforest anywhere - way overpriced, not very good food, and inconsistent service.