I have to disagree with cruiseman. I think there is a significant difference line to line and ship to ship in the areas of food, service and cabins.
In my experience - 93 cruises to date - Carnival & Celebrity have large standard cabins vs. RCI, NCL. The food and service on Celebrity is very good to excellent, RCI very good (usually, although I think Voyager class is a little weaker there) and Carnival and NCL average.
Nightlife is dignified on Celebrity and Princess, diverse and fun on RCI, and a bit more exuberant on Carnival.
Carnival and NCL are decidedly more casual as well. If champagne and caviar are your thing, I would not choose either of these 2 lines.
Ship decor by cruise line is also very different so if you like the glitzy, Vegas stuff Carnival is probably a good choice. If you are after a more elegant style, Celebrity and RCI Radiance class are a good choice and overall I like the open, lots of natural light and airy feeling of RCI vessels in general.