I agree with Lili that cruising is best in big groups.
I have been on two cruises:
1986, Western Caribbean= Bahamas, Grand Cayman, Cozumel and Jamaica.
2002, Alaska inside passage
Cruise ships have changed a great deal since my first trip in 1986. They were nice then but are amazing now. You will never be bored on a cruise. The rooms are a bit larger now and far more comfortable.
In 1986, we had a group of 26 people and had a blast.
To answer your questions:
From what I have observed most people leave the boat to explore.
In my opinion there are three main reasons people cruise. The cost of the trip is a known quantity ( except for drinks and tipping) and can often times be a bargain compared to a Caribbean Island. Secondly, it provides a safety net for people who do not travel much. They have everything they need on the ship and don't have to leave. They can meet new people and have fun on the ship. If they get brave they can get off at a port. And lastly, I believe many people use a cruise to look at several islands and decide where they would like to go back in the future for a longer trip.
All types of people take cruises. I am not sure you can find a "normal cruise passenger". Now you may be able to find several things in common with people who have done multiple cruises but that was not your question.
We have been to nine Caribbean Islands and two towns in Mexico on the Caribbean coast and I can tell you that each island has a personality of its own.
Beata and Tomek-- as for only older people taking cruises that is not the case at all. Many, many young people are out there cruising. You may find more "older" people on some of the more expensive boats like Holland America or Crystal but NCL, Celebrity and especially Carnival. Many of the ships today have themes which attract older vs. younger or the other way around.
I agree with Bob that the Inside Passage is a fantastic trip and you must consider doing it sometime. I agree that I would not want to take one in the Mediterranean.......but that is personal preference. We have friends who have done that trip and have been extremely happy. Again, I agree with Bob about the ports having many of the same shops. I will tell you each island has a unique personality and you can still feel the difference on your one day at port.
As Lili points out they are wonderful for families because everyones needs can be met.
In summary, I loved--- absolutely loved our cruise to Alaska.
I enjoyed the Caribbean trip but was left feeling like I needed more. There was not one port where we stopped where I felt like I had seen everything I wanted to see. It was like waving an exotic appetizer in front of my face and only letting me have a small sample. Since 1986, I have gone back to each of those islands to explore.
There is a cruise to South America that I am interested in some day. It would be a nice over view for the country.
I'd also consider doing the Panama Canal.