I agree what previous posters have mentioned, what makes a true luxury hotel is its service.
I have stayed at some really magnificent design villas in the past, but the level of service is in no comparison to hotels like FS or Aman, and that really makes a difference.
For truly wonderful hotel service, I expect the staffs to understand and accommodate my needs way ahead of me, and they should also fulfill reasonable requests in a timely manner. They also need to be professional but sincere, not being pretentious. All guests should be treated equal.

Location is important to me too. For a city hotel, it should be located in the most convenient or popular area, if not, a luxury hotel shuttle should be included in the price. For a leisure hotel, I'd expect my room to come with a breathtaking view.

Food is very very very important to me! Especially in a resort when you're pretty much stuck there most of the time.

Facilities should also be top-notch and high quality, though it doesn't really have to be some famous brands....
Bathrooms doesn't need to be marble, but should be bright and comfortable, even better if it comes with steam!

I also appreciate a large dressing table with bright lights, always a plus for female travelers I assume!