Every person is going to be different. It also depends on the cruise line and boat you took. Personally, I'm not a big fan of Carnival.
Cruising is really for two types of travelers. One, the kind of traveler that just wants to relax. A real vacation where you aren't constantly, you get pampered, good food, and just pure relaxation the whole time, with the option of going to a new destination, or just staying on the ship. And/or the traveler that wants to visit a destination before planning a trip. One day at port to a destination on a cruise can really be helpful in planning your trip, that way, you know what you are getting into and where you will be staying, etc.
I think the fact that you were sick may have put a damper on your experience as well. Any vacation you take where you don't feel good isn't going to be fun.
If you are an active person and want to be out doing things all the time, cruising likely isn't for you. But it's really intended to be an upper-class environment, with fine dining and a relaxing environment for people that want it. It's definitely not for everyone, but it does have it's place for certain travelers.
Personally, I love cruises. I've taken friends on cruises who loved it so much that's all they talked about for months. I've also had friends that just don't like it.
The cruise industry is really changing though, there is going to be a lot more to do onboard than ever before as the boats continue to change.