Matching your personality with the cruise is crucial. Nothing is more frustrating than to find that the cruise you have booked is nothing like your vacation personality (and is very different from the cruise you expected!) Here are some of the major considerations you will want to acquaint yourself with before you cruise:
Size of ship
Cruise line "personality" and fellow cruisers
Itinerary
Length of cruise
Sailing time of year
There is a good bit of difference between the experience of sailing on one of the megaliners and cruising on a much smaller ship. The larger ships will offer many more amenities--ice skating rinks, rock climbing walls, speciality restaurants and more are not uncommon--than can be found on smaller ships. In contrast, the smaller ships frequently offer a more intimate experience as well as closer--and more frequent--contact with staff and crew.
Although all of the cruise lines will be able to take you from Point A to Point B, the experience onboard can vary a great deal. Cruise lines do have personalities when it comes to things like dress codes, entertainment, dining and more. It makes sense to match the cruise line's personality (and the personalities of your fellow cruisers) with your own as closely as possible.
Choose an itinerary to match the type of vacation you desire. For example, if your goal is to "vegetate" for a week, you probably will be less pleased with an itinerary with that visits a new port every day. Likewise, if you are an active vacationer and want to see lots of new places, an itinerary with numerous days at sea will most likely not be a good fit.
Just as cruise lines tend to have a personality, cruises of different lengths will also have varying personalities. Nothing is written in stone, but frequently shorter (less than 7 day) cruises will attract a younger, more active crowd while on longer cruises (11 days or more) you will often find older and more frequently traveled cruisers.
You will find that there is a good deal of variance between cruises depending on the time of year of the sailing. For example, cruises during the summer months and during school breaks will be geared mostly to families, since there will be a good many children sailing at this time. In contrast, cruises that sail in mid-winter will frequently be more adult-oriented.