It's been a long time since I made the jump, but every now and then I'll do an U16 game on a free Sunday. The thing I'd imagine (other than obvious physicality) stands out is the sudden drop of skill level. Players at U16 can be quite competent, and when you jump "up" to your local league's Junior Division 8 on a Saturday afternoon, it can be quite eye-opening.
If you know any local refs you can go and watch at that level, join 'em for the day!
If anyone is Hampshire-based, always welcome to come find me on a game somewhere. :]