I mean, I don't know that "difference" between D1, D2, D3, NAIA, or anything else for that matter as I don't have any first-hand experience myself.
I would have to think the only real difference is the level of competition between the divisions since I don't think the NCAA would have a set of different rules and regulations for each one of those divisions. Everyone who competes at those levels are still considered student athletes where they would have to go to class and practice regardless of what level they play at. Now sure some of them have a free ride to do so, but that goes back to the level of competition.