As others have said the atmosphere is definitely more casual and the demographic younger and somewhat less socially refined than you will find on Celebrity. We have friends that absolutely love Carnival and have cruised with them a bit. For reference we have sailed with Celebrity 3 times (one more booked), HAL 4 times, RC once and Carnival 7 times (one more booked.)
We enjoyed ourselves on every cruise. The only altercation we every experienced was a VERY drunk lady attempting to get back on the HAL Eurodam in St Thomas being very abusive to the staff who were only trying to assist her. She and her husband / SO / whatever got to remain in St Thomas. The more questionable pool "contests" on Carnival (Hairy chest, wet t-shirt, etc, etc) are no longer done and the pool events now are not as crass as the past. We avoid the main pool and tend to spend sea day time in the Serenity adult only areas on Carnival.
Food is super subjective but in our opinion MDR food quality, in order is X, HAL (Very close), RC then Carnival. Buffet HAL, RC, X, Carnival. Specialty, X, HAL, RC, Carnival. Casual food, Celebrity then the other 3 with a sizable gap.
We found RC had the best shows and entertainment, Carnival shows were OK but the Punchliner Comedy Clubs are great and they had some good smaller groups spread out around the ship, X shows and other entertainment was consistently good, HAL was a mixed bag, some great entertainers and some ho-hum.
Overall I would not hesitate to book any of the 4 given the timing / itinerary / price met our wants / needs.