For us it was definitely Artists Point. We were so diappointed after hearing nothing but rave reviews. Our server was terrible, she wouldn't leave us alone, untilo she brought the food. Whle DH's was excellent, my steak ordered rare was served burnt to a crisp well done. Seriously overcooked, you could tell by looking at it. I sent it back, it was inedible. Even DH who will eat his steak cooked as much as medium well said there was no way he could have eaten it.
So I sent it back. The next one came out cooked medium, and on the medium well side of that. I had ordered it rare.
It was very disappointing.
We also had a bad experience at Portabello. The a/c was broken, and they sat us in a room that couldn't have been more than 60 degrees. We were so cold we had to leave half way through our meal, The server was terrible as well.
Other bad experiences...
Palio. The first time we ate there still ranks as one of the best meals/service we've ever had at WDW. We went back a year later with a party of six, the server was terrible and the food lackluster. Chocke dit up to a bad night and returned a year after that. What a mistake. The service was terrible and my sons chicken parm was served without sauce.

Asked the server for sauce, she disappeared for half an hour. DS finally left to get a hot dog on the Boardwalk. We won't go back.
San Angel Inn, we went not expecting too much. What we got was below our already low expectations. Server took the order, we never saw him again. Terrible food dropped off by someone else. Over half an hour later we finally asked the hostess to find out check.
Le Cellier, last summer we had a terrible experienc with poor service and not so hot food. Chocked it up to a bad night, we've eaten there a dozen tiem prior and always been happy.
Others that weren't horribly terrible but I won't bother to return:
Tony's, lackluster service and food, not at all worth the price.
Pinnochio's Village House, terrible food, not enough seating.
Big River Grill, so-so food, so-so microbrew, and so-so service, so-so many better options.
Sci-Fi, been there, done that, not worth going back.
rainforest cafe, decent food but a bit expensive for the quality, pretty poor service.
Planet Hollywood, so-so food, so-so service, not worth spending vacation time to go there--basically a tourist trap.
ABC Commissary, food poisoning. DH and I ate the same thing, the next day both we so sick we couldn't get out of bed for two days. DS had something different and was fine.
Anne