WOW, a tough question to answer in a few sentences. We fell in love with cruising 50 cruises ago. And, every once in awhile we take a trip to Europe or go on some other land type vacation and almost always end up saying "We could or should have gone on a cruise!"
We like all the others who have replied like every aspect of cruising. We find it the most relaxing vacation we can take, virtually stress free.
There are lots of things aboard ship that we don't do, like bingo, the casino, etc. but, they are there should we decide we want to give them a try. Most of all I think we enjoy the people we meet on every cruise. We spent three weeks driving through Europe and I can't recall a single person we "met" during that trip.
With cruising we get "No Surprises." And as others have said it is the best vacation value for the money.