Can Rees run an offensive practice? I know Kelly will have input, but ND has ran about one successful screen pass in 11 years. There is a certain level of execution that isn't translating from the practice field to the games. New ideas, habits, drills, emphasis, etc are needed.
Can Rees manage a staff? I know the staff might like him, but that means nothing if they don't respect him. Means nothing if he can't or won't be critical of them when needed.
A book will be built on Rees. Can he adjust as DC's start figuring him out.
What does Rees do when the game plan and scouting is trash? It happens to the best coaches, everything you thought would work doesn't. The team does something completely different than you anticipated. Great coaches can go to the drawing board or pull out of memory previous situations that were similar and remember what worked. Quickly.
Can he handle the pressure? Sure he was QB at ND and that helps, but now he is getting paid some serious money and that brings an entire new set up pressure to the table.
I'm sure I've missed others, but as west cost said, there is a lot more than calling plays and scouting a team.