So I'm back from my trip, and I can report on cruising solo with NCL.
1. Having a balcony stateroom to myself was amazing. It was comfortable and I can't imagine trying to fit 4 people in there, but as one it's plenty of room.
2. NCL is supposed to be really good for solo travelers with activities, but I found the main ones were going to dinner and in the evening together. Not much going on outside of that in terms of meeting other solos.
3. As a result, some of the sea days had a lot of downtime. I wish I had brought a book, some movies, or anything else to pass the time during the day. This was an Alaska cruise, so I didn't spend too much time swimming. I know Disney does a good job of filling in these days with character meets and daytime shows/movies, plus on a 3 night you usually don't have sea days.
4. While they did have some daytime entertainment, it was almost all gambling-related or trivia, and those just don't appeal to me. Same with most of the gameshows at night. NCL had 3 nighttime shows performing multiple times per cruise, but Disney might be lacking here.
5. Dinner and the shows were great. The servers do a good job of pacing the meal when you're on your own so you don't sit around bored but also aren't rushed. On NCL I always had my own table, but with Disney you'll probably be placed in a table with other adult groups. They shouldn't put you with families with kids.
6. I really want to try Disney to Alaska, but I think I might bring a friend or go as a group.