I think bad players make better corches. First, I think a good corch does need that deep knowledge of the game, so experience helps in that regard... Second, I also think the job of the corch is to find a way to schematically put all of your players, even the ones who aren't that good, in a position where they can be successful. Being a failure as a player helps you have empathy for your shithead players and makes you more likely to find a way to help them succeed than to just assign them some hard task and get mad when they fail. Third, beyond scheming, you also have to be a good teacher in order to actually have your corching stick.
Manning obviously has the football knowledge, but to me I think he really would be lacking in the other two areas. He's such a perfectionist in hisself that I just don't think he'll ever understand the fact that sometimes his players aren't capable of accomplishing something he asks of them (GOD DAMMIT DONALD!). I also don't think he's really that good of a teacher either, as evidenced by the fact that he spends most of the game by himself pouting on the bench and really has never seemed to want to tutor younger players (or even allow them practice reps or reps in a blowout). Maybe he would be a great corch, but I don't think it would be an automatic transition like most people assume, even as an offensive coordinator.
Lol
@Bdub saying he seems to work well with others. I don't think I've ever seen a player isolate hisself from the rest of the team more than Manning does. It bugs the shit out of me. Reminds me of Cutler, but worse, lol.