Must be quite exciting to get back to sailing after such a long break. Anyways, royal versus carnival is a discussion I could go on for days. In short, here are some of the finer points:
1. You've been away from sailing for a while, so don't expect the same caliber of service in general across all lines. It quality of services as ships got bigger appeared to have degraded.
2. Carnival doesn't pride itself on it's amenities so don't expect a rock wall or ice skating.
3. Carnival does tend to have much better food quality as opposed to royal.
4. Expect a more social atmosphere; carnival tends to be a lot more of a party hard line than royal.
5. Expect less people. The valor weighs in at 100,000 GRT, meaning she carries around 3400 double occupancy. This is significantly less than royals average number of people.
6. From a teen perspective, no curfew! That's right, carnival does not have a 1 am curfew.
7. More deck events. Carnival does a lot of family things on deck, and their deck parties are some of the best at sea.
8. You can still expect most amenities found on cruise ships, and some newer ones (movies by the pool).
These were the major ones I came up with. Let me know if you have any more specific questions, and i'd be glad to chat with you about the two lines.