You should avoid the ships with "World" in the name. They are newest, but like Celebrity E-class Retreat, YC space isn't big enough to handle so many YC cabins. I sailed Meraviglia, which was great. I understand that anything with Sea* in the name also has ample space per YC guest.
As far as the food, I found the ingredients to be of a higher quality than on Celebrity--better fish, better beef, lobster tails of an acceptable size, etc. YC also has their own buffet, obviously smaller than the main buffet, but plenty of choices.
As far as your "packed Carnival cruise ship" comment, I would say if you leave the YC area, you also feel like you're on a packed Celebrity cruise ship, but with more activities going on. These activities create some crowded, but completely avoidable, public areas. We left YC on occasion to have drinks at the champagne bar and the Irish pub style bar. Both were great and never crowded. The real crowds are found on the pool deck and in the main buffet, both of which there is no need to visit.
All in all, what I wrote upthread was not hyperbole. The YC experience absolutely blew away any Retreat experience I've had.