I've argued the point for years when people pimp this assistant or that for a head coaching position. A lot of them work out. Great examples with Stoops, Richt etc. A lot more don't. The difference in responsibilities for a HC and an assistant are worlds apart. The assistant deals with his assigned duties for the position he coaches, recruiting and so forth. The HC has to hire and fire, oversee the staff, decide how, when and where to practice, organize his sideline operations/communications, how will we walk out of the frickin' tunnel? He has to do playback shows, commercials, speak at every fundraiser, QB clubs....deal with the administration...Jay Jacobs and sometimes..the infamous BOT. On top of that, every step you take, every meal you eat, every dump you drop is talked about, analyzed and rehashed in every sports media outlet, message board and coffee shop in the State.
In other words, when you consider an assistant as a HC at the D1 level...especially in a BCS league, his playcalling abilities are probably way down on the list of requirements. I would imagine it's quite easy to get in over your head before you know it.