I Like Zimmer as a "leader of men" type and as someone who has a clear vision of what he and the team will be about. God knows Ross and Ireland are not good at communicating and inspiring any of us. Zimmer is a straight shooter and our team may take on his fighting spirit. His ties with Ireland are a good thing and again, I don't see him backing down on any point the way some of the lesser experienced OC may if there is a disagreement about a player etc. Zimmer could keep Nolan around and bring in Haley as suggested earlier, although it's difficult to look past the solid work of Dabol over the last half of the season.
McCoy, I don't know as much about, but I've read that he is articulate and people gravitate to him, but can he lead a team? Time will tell. Nolan will probably stick aroud with him as HC as well.
The biggest thing with all of these guys is who can put the best team of coaches together and you need experience in the NFL to do just that. In the end that's why I hang my hat on Zimmer and take the next step...