We've only been on Carnival and Princess. We prefer Princess, but wouldn't necessarily rule out Carnival in the future. It's just that Princess has longer cruises than Carnival (a plus for us) and itineraries that we prefer.
One thing that would keep us from sailing on some other cruiselines is that we want to utilize self-service laundries, especially on cruises lasting more than a few days.
And now that we have experienced flexible dining and decided we didn't care for it, we want a ship that has the traditional experience.