Some answers about the dining onboard Queen Elizabeth 2:
Mauretania is the "entry level" restaurant with two seatings in the evening. Food is supposed to be 4 stars level. Mauretania has its own galley.
All other restaurants are single seating. Sometimes the menu in the Caronia restaurant is the same as in Mauretania, sometimes some additional items are on.
Caronia and the two restaurants on the next level, Princess and Britannia Grill, share the same galley. Yet, in the grills you get not only more choice on the normal menu but also an additional a la carte selection. Even more, you can ask, if early enough, for many items off the menu.
The pinacle of gourment dining is the Queens Grill. It has its own galley and has been rated many times as best restaurant at sea. Order what ever you like.
To some up, many items sound like being the same in all restaurants, but choices increase as do attention to detail, presentation, the number of waiters per passenger, service and consistency of quality.
One example for differences: You get only a pre-prepared cheese plate in Mauretania, while there are cheese carts in the other restaurants.
It's luxurious in all restaurants yet there are worlds between Mauretania and the Grills.
My major objections against the Mauretania are the rush due to early and late seating, the inconsistency in service during open seating for breakfast and lunch and the sometimes really limited choice. Yet, that's of course what passengers are used to on most ship these days.
In the end, many people are more than happy with the Mauretania food, given it is still better than those on most ships.