Carnival is more kids and family oriented.
Royal Caribbean is less so.
Open bar - no such thing on a cruise. Shit you have to pay for everything except water, crappy breakfast coffee, and iced tea.
True about rooms without decks being much cheaper, even cheaper than that is a room in the middle of the boat. But you get no window at all and the rooms are smaller. I much prefer a room with a view.
Prices for longer cruises in the Caribbean are not that bad, from 3/4 to 5/7 days. I just got back from a western Mediterranean cruise and going from 7 days to 10 was like double the price, which was already like 5 times that of a Caribbean cruise.
Best bang for the buck happens off the boat most of the time. Snorkeling in Cozumel and Cancun is really nice. If a stop at Aruba, you can snorkel at shipwrecks.