Will they have apps, sub-accounts, lists of favorites, and be able to move from device to device and pick up where you left off?
I have to guess that Disney's depth of content is pretty shallow. You have kid movies, older Disney content, and then franchises they bought like Star Wars and Marvel. But those account for a handful of movies a piece. Maybe it won't be hard to technically serve that content up since it is much smaller than Netflix' library.
What about content delivery? The technical part of this is probably a very hard problem to solve, globally, where people's streams aren't interrupted or dropping frames, etc.
Just small things like when I fast forward, Netflix shows thumbnails of the content, will Disney have those small quality-of-life types of features?