Some of the most luxurious cruise ships in the world are also small ships. Luxury cruises on a small ship (compared to a large luxury ship) has many of the advantages we expect from small ships in general - we can visit ports that the big ships can't, there is an extra level personal service just becsause the ship carries fewer passengers, and many of your experiences are more up close and personal than they could ever be on a large ship.
There is no hard and fast rule that defines a ship or a cruise line as luxurious. We have used a subjective evaluation of service, accommodations, food, and, yes, price to arrive at the lines to include in this category.
The Yachts of Seabourn - subjective or not, there is little doubt that Seabourn belongs in this group.
Seadream Yacht Club - is a relatively new cruiseline but their orientation is definately luxury.
Azamara Club Cruises - covers the globe with interesting itineraries with a luxury orientation.
Silversea Cruises - worldwide itineraries with luxury orientation.