I will list things I think Carnival (and the other lines) do better than
DCL.
1. Anytime Dining - the other lines all have the option to choose to have dinner in the MDR at your leisure, DCL is too regimented
2. Sitting alone in the MDR - See above, with anytime dining you will be seated as a family, you do not have to share a table and make small talk with people you may not like
3. Included food options - CCL has Blue Iguana, Guy's Burgers, Pig & Anchor, Cucina del Capitano, the other lines have things like O'Sheehans, Noodle Bar, etc, DCL only has the buffet and MDR for everyone (it looks like they have a small cafe for adults)
4. Split beds - All rooms have a bed that splits into twins so if you are 2 adults traveling together who are not married you can have your own sleeping place in a real bed (not a sofa)
5. Fun stuff - Waterworks, rope courses, Sky ride, roller coaster, climbing walls, ice skating, many things to do if you want an active vacation, DCL I believe has 1 slide on only 2 ships
6. PRICE - I can take 2 vacations a year for less than the cost of a 7 day cruise on DCL