I think a lot of it has to do with intinerary, too.
We encountered nothing but classy people and a wonderful time on our Carnival Spirit Alaska trip. From the reviews I've read, most any Caribbean or Mexico trip can get out of control depending on time of year and who's on it, but other itineraries draw less of a party crowd.
I did some research for a family trip to Alaska next year and compared some ships. One of the comparisons was 'normal' passenger to crew ratio. Carnival's Spirit is the only one that goes to Alaska, but the p/c ratio was good, better than some others even.
All ships try to sail as full as possible, which is why you can get some great last-minute deals.
Anyways, I've only been on that one cruise, but it was fantastic. I'll let you know how the Ecstacy does in Mexico on Thanksgiving.