It really depends on what your most interested in doing(dinning, night life, pool side relaxing, shows & games..) Here is my take on the different cruise lines....
Carnival- most of the time the cheapest rate, very diverse group of cruisers, mostly middle aged couples or families with kids. Emphasis on fun, silly entertainment (hairy man contests, conga lines ect. ) Ships are designed in a classic vegas style.
Royal Caribbean- ships are a little nicer than carnival, clean-lined and natural colors. Emphasis on entertainment, particularly physical activities. Again diverse group, but mostly families.
Norwegian Cruise- There ships are brightly colored! Emphasis is on lounges and dinning. They normally have 8+ dinning options, some are at an additional cost, but surely something for everyone, plus your normal main dinning. I've heard they've got one of the better entertainment shows. Cruisers very on diversity depending on itinerary and time of year.
Holland America- very classy ships, decore is very elegant, you'll see a lot of wood finishings. This cruise line has stayed close to the old "traditions" of cruising (tea times, formal dinning, shuffleboard, spacious rooms) Emphasis is on service and quality. You'll see an older crowd on these ships. 40s+, however you'll still see some families and youth on board.
Celebrity- a bit pricier, however stays with the clean lines similar to Royal Caribbean. Again you'll see less families on this cruise line. Emphasis seems to be on service here. I always recommend this to couples, particularly honeymooners!
Princess- There emphasis is mostly in catering to senior passengers. They offer great cruise tour pkgs. (always recommend for Alaska!) They offer anytime dinning similar to several Holland America ships and Norwegian as well as traditional dinning. Decor is elegant, but not to the same degree as holland america.
MSC & Costa- Are Italian cruise lines. Very popular in europe. Often you'll see a kids sail free with them. Costa's Europes version of Carnival, however a little bit more upscale. MSC has an older crowd than Costa and often times you'll see more europeans on these cruise ships. Both emphasis on italian experience and majority of staff is Italian or European.
HOPE THIS HELPS SOME! IF you'd like to see picture comparisons of the ships go to... www.myspace.com/goclassycruises