Besides soda I can't think of anything that's included on DCL that isn't included on RCCL. Maybe I'm wrong though and forgetting something.
On DCL, there's interaction with characters that actually talk. The character breakfast is free for kids (The Royal one is not that great, so you're not missing anything tbh). But I think that, for the most part (and this would go for other cruise lines as well), there's really not much difference in the "free" amenities.
There's also the rotational dining, but realistically it's not that much different from having different daily meals in the same dining room.
Perhaps a significant thing is that on DCL, you can bring a 6-pack or 2 bottles of wine aboard per person, per port of call. On Royal, you can bring aboard 2 bottles of wine per stateroom (not strictly enforced) during initial embarkation only. DCL does charge $25 for corkage, Royal $15, although on my cruise in August they didn't charge us. YMMV there.
Kids clubs are slightly different. Adventure Ocean (usually) splits the kids into groups by age (3-5, 6-8, 9-11). Registration process is much the same as on DCL.
Not to hyjack this thread but I have gone back and forth with wanting to try RCCL and can't seem to do it. My main concern is all the extras onboard that cost money. I liked not having to say no to my kids on Disney. Everything was included. Yes there is a gift shop and a couple other things that cost money but not much. What cost extras on RCCL? From what I have researched and I could be wrong because I find their website difficult many of the food options are extra, drinks, kids activities.
So here's the thing - I think there are
more opportunities to spend your money on Royal vs. DCL. If there's a sale on the Promenade, you can't help but notice it, whereas there's a chance that you may not even get within breathing distance of the shop on DCL. DCL has only two specialty restaurants, while Royal's boats, generally speaking, have AT LEAST two specialty restaurants. So most definitely, a higher percentage of food options on Royal do cost extra (compared to DCL), but don't think of it as "MDR and buffet are free, everything else costs money".
There are, of course, the ubiquitous drink packages. I think DCL might be the only cruise line that doesn't have an unlimited alcohol package. It is what it is.
And let's not forget the casino - perhaps the Exhibit A of places on board that costs money. I like (non-smoking areas of) casinos, but I do applaud DCL for not having one on board their boats.
How do the shows work? Do you have to plan ahead and reserve tickets? Are they hard to get?
With shows, if you're on Oasis class you'll be able to book showtimes online about 90 days out from your cruise (works like FastPass+ in that regard), and for those there are at least 3 times to choose from. There may be other shows that are available on a first come, first served basis.
The website can be a pain, btw.