Of the three mainstream cruiselines (NCL, Royal and Carnival) NCL is on average, the priciest and more upscale of the three. While i agree with most cruisers in that they are more similar than different, and different ships/itineries in the same cruiseline can differ drastically, there are some major differences in how all three market themselves.
NCL charges more for their rooms but have an emphasis on making their passengers spend A LOT while on board. This is backed up by data that shows an average NCL passenger spends about 50% more on board than on Carnival, which focuses on cheap fares to bring on as much passengers as possible to compensate. Both these plans work in their own way, as Carnival is the largest of the 3, but NCL being the most profitable.
I've never sailed on Royal, but they have always stayed somewhere in the middle of the pack for most metrics. Theirs ships, however, are the largest by far...
This strategy therefore attracts different crowds. Carnival cruises tend to be younger and rowdier as the cheaper fares attracts families and people on a budget (college kids), while NCLs higher prices are restrictive for some.
On a personal note, I love NCL! Its my first choice of cruises. I proposed to my wife in Bermuda while cruising on the Dawn! Great memories. I think the extra cost does translate to better food, better service, and a cleaner and quieter ship. I will say that I am dismayed that NCL is getting rid of their Broadway shows and not replacing them with anything! ANYTHING at all would be nice... This and the fact that they started charging for things like mini golf on their larger ships is disappointing.