First----Itinerary. For some time we went for the exploration type voyages where we were able to visit or explore exotic locations or those parts of the world we never dreamed we would get a chance to see. As time goes by we have seen a lot, so now the best itineraries often have ports included that we have been at multiple times. And while getting older our tastes change somewhat too we have began to look for many sea days and as a result are more and more getting away from our obsession to only sail on the smaller vessels. There are some features on these new large ships that appeal to us now.
Second---The reputation of the cruise line. Cruising is changing so rapidly that one has to really pay attention. We have been hard core HAL and Princess fans as well as the all inclusive Regent. However, Princess and HAL (HAL in particular (smoking mainly), are demonstrating a few negatives making it increasingly difficult to stay loyal. Last cruise on Princess the MDR experience was not tolerable. Regent was always expensive initially but over the course of a voyage it wasn't that much more, however, it does appear they are beginning to jack their rates at the same time decreasing some of their service and facility features.
Third---Ship. But that pretty much goes along with the lines reputation, our experience has been if one ship is pretty good they all are.
Fourth---Price. Never really had much problem with that before as it seemed the overall costs were commensurate. But last fall we really got a shock when we decided on Regent only to find out they were almost three times the cost of an equivalent cruise on Princess, albeit other charges would have to be factored in for Princess. However, in that case, the difference was around 25K. So, for that kind of dough I can buy a lot of shore excursions, wine, speciality restaurant meals on Princess, while staying away from their MDR, and still have a lot of change left over.
So----Right now we are taking a very hard look at Celebrity and also at MSC--if MSC comes up with more selections beginning or ending in the USA. Hate to give up all the perks on the other three though, it has taken many years to earn those.