Just returned from our 7-day Conquest cruise. I was a former Royal Caribbean cruiser and was a little concerned with Carnival's reputation as a lower quality cruise line. Unfortunately, it is a reputation that is deserving. I also have a few tips for future cruisers who may be able to use them.
The two major differences between RC and Carnival is appearance/beauty of ship and food/service quality.
The RC ships we sailed out of Miami had huge open Promenades that had soaring ceilings for nearly the length of the ship. Being a little claustrophobic, this was huge for me. I never felt like I was too closed in. The Conquest ship has a very small atrium and the remainder of the ship has really low ceilings and tight walkways so that it gets really congested when everyone is out and about. Our cabin was great. It was a Jr. Suite. At almost 2 grand a person, it was pricey, but it was really roomy and I need the extra space.