Diamond Horseshoe only opens when they need over-flow dining options (major holidays, for example, when the regular restaurants are fully booked)
For lunch they run it as a limited-menu cold food counter service...basically chips & sandwiches. For dinner, it is NOT a buffet...it's served family-style, and the meal is exactly the same dinner meal as Liberty Tree Tavern (which is where the food comes from--the Liberty Tree Tavern kitchen. The meal resembles a Thanksgiving dinner with turkey, ham, etc.)
"family-style" is a set menu, no choices: the server brings a bowl of salad to table,a basket of rolls, a bowl of mashed potatoes, a platter of meats, etc. You can request more on any part of the meal, but it is not a buffet line with multiple choices.
The show part has unfortunately been gone for years...it's a neat looking place & would be awesome to turn it back into a show venue. Even if they just did little 15 minute shows it'd be fun.
Editted to add menu: Tossed mixed Greens with Strawberry vinaigrette, dressing, Roasted Turkey Breast, Carved Beef and Pork Brisket (tastes & looks like ham) with Mashed Potatoes, seasonal vegetables (the seasonal vegetable was green beans last time we ate there), herb bread stuffing and Mac and cheese. Rolls & butter. Apple crisp with Ice cream for dessert (arrives in a big bowl with the ice cream on top)...that list is from wdwinfo, and matches my recollection of our meal.
From Disney's own website, it says "Diamond Horseshoe at Magic Kingdom theme park serves American classics in an all-you-care-to-eat, Western-themed restaurant located in Frontierland. Savory entrees like carved beef and smoked pork loin are paired with tasty sides like herb bread stuffing."