If "adult" means no characters, and more adults present than kids, then I second the vote for Kona (Polynesian HOtel) or The Wave (Contemporary) or The Grand Floridian Cafe (GF hotel of course.) All of these places do have families dining, but not near as many kids as the buffets, so definitely quieter. although Kona can get a little loud because it's open to the Ceremonial House lobby, so the bar noise & the noise of people waiting for Ohana' can get noisy in Kona.
For really elegant looks, go with the Grand Floridian Cafe...and get the sampler for dessert. For "new age" peaceful & upscale looking, try The Wave (great steak, incredibly dessert trios)
All three are 1 tableservice.