We cruise a lot of lines, including Carnival, but this cruise was definitely among our least favorites. The 3 star rating is in light of the fact that we enjoyed some of the entertainment and the ports. Also, the weather was wonderful, so it was a nice getaway.
First, to us, the food and dining experience was definitely not up to the usual Carnival standard. Many of the dishes were quite bad (looking at you, Penne Mariscos). And the MDR service was the worst we've had on any ship. You had to ask for everything and that didn't mean you'd get it. The biggest struggle was getting coffee to go with the dessert course at dinner - I think due to the fact that you now order dessert with the rest of the meal so it's just another thing for them to bring out. However, one breakfast we had a very good waiter, who reminded us of the better Carnival service we've enjoyed on other ships.
The food in the buffet was somewhat better, but as others have commented, there wasn't much variety and often long lines. The pizza was quite good, but I don't eat meat and they seemed to make mostly pepperoni. The one time I was able to get a cheese slice, it was really good. Also liked the sometimes-available gelato.
The bar service was generally good and friendly - especially in the Ocean Plaza. We scored a great deal during happy hour in the Red Frog pub one port day afternoon - a pitcher of mixed drink (forget which one) for just over $11 including gratuity. We drank it in one of the nice outdoor seating areas on Deck 5 - one of the ship innovations that we liked.
We really liked the Ocean Plaza as an entertainment venue, and especially enjoyed the bands Studio 6 and the Ambrassadors (who also performed in the Rock production show). This is where they also did trivia, dance lessons, etc., and you could also get surprisingly good breakfasts and lunches there. It was also around the corner from the coffee bar.
We also tried the light lunch at the Serenity, but it turned out to be just salad (although with lots of fresh ingredients), and it was hard to get a place to sit. The Serenity on this ship seemed small and crowded.
Our inside cabin was fine, and our room steward did a good job. As people have mentioned, the elevators were a pain. Too few and too slow.
We have another Carnival cruise booked and expect to add more - just won't be the Vista.