I'm new here, but I thought I would answer anyway.

My first (and so far, only - though I'm preparing to book number 2) cruise was a 4-night cruise on Carnival.
At the time we booked, I had exactly one demand of the cruise we chose. I had a long-time fantasy of having a Christmas at sea. So, when we finally decided to actually take the plunge and try it, I was ONLY interested in cruises that were "at sea" or "cruising" on December 25.
The actual number of available cruises that met that very particular requirement was very small. We eliminated cruises that left from the other side of the country or other countries, and the number of potential cruises got even smaller. We eliminated a couple of other cruises for various reasons that I can't recall now. Finally, we were left with just two choices that we were seriously considering. One was the 4-night cruise on Carnival, and the other was a 7-night cruise on, um, I think it was Royal Caribbean (but I won't swear to that). It was a very tough choice between those two, though.
There were several factors in us deciding to take the shorter cruise. Some of them included:
- the shorter cruise was cheaper
- the Carnival ship had a WATERSLIDE!
- the Carnival ship stopped in Cozumel, where we could do a DOLPHIN SWIM on Christmas Eve!!!! (a dolphin swim was another one of my fantasies, something I had wanted to do since I was 4 or 5, and to be able to fulfill two fantasies for Christmas - one on the day, and one on the eve - made the vacation seem like it would be magical)
But the number one reason we opted for the shorter cruise was because my spouse worried that one of us might get terribly seasick. We reasoned together that 7 days of being seasick would be mighty miserable, but 4 days was actually bearable.
It never crossed either of our minds that this vacation could possibly be anything BUT magical and wonderful in every way - UNLESS we got seasick, of course.
Well, I can tell you without the slightest hesitation that the cruise WAS magical. The dolphin swim was incredible; I only wish I'd booked two *grin* The ship was great in every possible way. There was so much to do on board that we didn't get to do it all - in fact, one of the major draws, the waterslide, resided in a part of the ship that we never found because the thing was so big that we didn't have time to explore all of it. The entertainment was spectacular. The staff was excellent.
There were a lot of nice people on board; most of the other passengers were really great. We only ran into one unpleasant person; at one lunch, we got seated next to a horrible man who spent the entire lunch telling his daughter (who looked about 7 or 8 years old) what a stupid, worthless person she was. (I dunno; one hopes he was just having a bad day, but I felt sorry for the kid all the same.) Other than that one bad experience, though, the other passengers on board were just fabulous. We even made a few friends we still keep in touch with.
I suppose the ship had a "festive" atmosphere, but I did not think that the partying was excessive. (Of course, my mood as an enthusiastic first-time cruiser could have affected that.) There were people of ALL ages on the cruise, from young children to old people who remembered the world wars.
How did I feel when the cruise was over? Why, I most definitely did NOT want to leave the ship. I was NOT ready for the cruise to be over. I actually wondered what would happen if I hid somewhere until the ship left port again
On the OTHER hand, I had never been to Miami before. So, to extend the vacation, we started out by booking a room at a resort called Don Shula's for four days before the cruise. As a result, we got to tour Miami and experience the unique and truly wonderful Don Shula's as well, so we had a full vacation.
Would I recommend that anyone else do a shorter cruise? Why, certainly. Yes, it's true that 4 days seems way too short, and that's definitely a minus. But, at the same time, 4 days is plenty of time to enjoy the novelty - and not too much time if something goes wrong and you don't wind up liking your cruise. (You are, after all, pretty much a prisoner on the ship until you return to your port of call, unless you care to get off at some random port and figure out how to get back on your own.)
Of course, I would also recommend that you pair your first short cruise with something else either before or after, if you've got the vacation time and money to do so. And, if you haven't got the time or money to do so, then you probably can't do a longer cruise, anyway *Grin*
Hope that helped just a little.