Cautiously optimistic. We don't need another Kerry Coombs.
I look at it somewhat similarly.
Maybe Coombs would have been a good defensive coordinator if he had cut his teeth doing that some place else than at Ohio State? I don't think he (or Day) fully grasped the reality that Ohio State always has a big target on its back -- Every opposing offensive coordinator will show no mercy whatsoever if they feel that Ohio State has a weakness tactically or strategically. A win is a career highlight on their resume.
This is where I'm hopeful-but-somewhat wary of Hartline being the OC: On the bright side, there's the fact that he's probably the best young position coach in all of college football. On the darker side, he will need to show that he can play chess against (and continually outwit) multiple seasoned defensive coordinators.
What I hope happens is that Hartline already understands this and becomes a great manager, instead of an instant hero. Leverage the wisdom on hand (e.g. Day, Frye, Alford and every other veteran coach) before diving into the deep side of the pool.