We sailed RCCL in August. So not Disney, but I suspect many of the effects of reduced capacity (both positive and negative) were similar. We had just under 500 passengers on a ship with a passenger capacity of 3500.
There were positives, as others have pointed out. No waiting for anything, obviously. On the one occasion that the rink was open for skating, we had it almost to ourselves. Nearly unlimited time on FlowRider. Window seats in the buffet at every visit.
The best was at RCCL's private island. IMO there is no downside to empty beaches and quiet pools, and no waiting at all during our time in the waterpark.
But... there were about 40 kids on board, split evenly (ish) between the teen and kids clubs. There were few organized teen activities, because there were just not enough kids to support it. My kids did go to the club the first night, and met a few other kids that they hung around with off and on throughout the cruise, but they rarely spent any time in the club itself, and when they did, it was just to chat with the staff.
Organized family and adult activities were also limited, and often not well attended. Okay for things like trivia, but not so great for the "game show" type events. Hours for some amenities and activities were also shortened.
Staff were awesome and certainly attentive, but at times it verged on hovering. We often found ourselves answering the same questions repeatedly.
I am not a lover of crowds, by any means, but for us, there is such a thing as too quiet. We had a great cruise overall and I would definitely do it again, but we will be hoping for something closer to normal by the time our next
DCL cruise comes around next summer, even if that also means normal crowd levels.