Most head coaches have their forte. Gailey's is offense. It clearly showed last year in the Bills' offense and if anyone is unconvinced of their improvement, that's ok- there are plenty of articles and opinions and a pristine offensive track record for Gailey that would speak to his offensive prowess despite one year's performance.
I think that the "genius" label on head coaching as a whole is a bit unfair. That being said, I think it's safe to say a head coach's ability to appoint the correct coordinators is a great factor in their performance. So far, Gailey has completely switched the defensive system and hired a mediocre George Edwards to coordinate it- but that was what he had available at the time. Since then, he has hired defensive guru Dave Wannstedt (another title that shouldn't need to be proven, he has a well-known defensive track record) and made it known that his run defense needed bolstering (which happened over the offseason- at least on paper). As far as offensive coordinator, well, his only best option was himself.
Taking the above two points into account, it seems that head coaches are just really good staff-pickers that can make executive game decisions and effectively motivate. I think it's clear that Chan Gailey is a motivational, firey coach. However, he's also honest (which I personally appreciate), unlike Rex Ryan, whom, while I respect his previous two seasons, I can not get behind his claiming that his defense-heavy, high school quarterbacked team is going to win the Super Bowl ever year...That's gotta start getting old in the locker room- maybe this year? I'd rather have Gailey who comes right out and says when he doesn't think we're good yet, but that we can be- I think that's a challenge to naturally competitive professional athletes, which yields performance.
So that makes Gailey a motivator/head coach attitude with sound coordinator decisions, equaling an average NFL head coach. Now the genius part (I know I already said I don't support the "genius" thing, but most do)- well, every genius is an average head coach, then they win a Superbowl and they're considered geniuses (see Tony Dungy, Bill Belichick, Mike McCarthy, Mike Shanahan, Jon Gruden, etc.)