Well to start with Welcome back to the world of cruising.
It is hard to compare lines, my true feeling is all mass marketed lines are more alike than different. the main difference between NCL and Carnival are 2 fold: 1-the freestyle dining adventure and the most subdued night life. This isn't to say it doesn't exist on NCL, it certainly does, but you won't find as many activities after 11pm or as many people wandering around the ship like you do on Carnival.
Carnival cabins are a little larger than NCLs and the ships glitzier but I find them boardering on gaudy. Again, this is my opinion.
Of course food is very subjective, I have noticed on this board more people prefer the food on Carnival, on another very busy board it is visa versa. It all depends on what you are looking for. The dining choices are one of the selling features of NCL. I happen to think the sur charge restaurants are better and less expensive than on most other lines. Again, many may disagree on this one.
Has cruising changed? Yes and no. I don't think as much in the past 4 years as it has comparing cruising now to 15 years ago, but yes, their have been changes. First of all, because pricing has remained pretty much the same and cost of operating the ships has sky rocketed you will find more things with extra charges. The cost of Bingo has shot up, same with photographs and liquor. Of course the auotmatic tipping policy is new since your last cruise.
I hope this answers some of your questions, if you want more details many of us will be glad to answer your questions. BTW, I have not cruised the Dawn but have sailed 10 times with NCL>