On my last trip to WDW (October), we ate at Jiko's, Flying Fish, Citricos, Naroossee's, Cali Grill, Le Cellier, Brown Derby, and Artist Point. The only place where there were any "restrictions" was at Cali Grill. As the PP said, the two most expensive sushi platters could not be ordered as appetizers, but they could be ordered as an entree. We did not run into any entree restrictions at the places we went to, nor did we run into any other appetizer restrictions.
Though, at some of the non-Signature places where the appetizer or dessert said "for two", there had to be two folks using dining credits to order that item. I remember the server specifically pointing this out at Via Napoli and at the Grand Floridian Cafe.