The way I look at it, the one time in his career he really had good personnel, he went 13-3. The two seasons out of his career where I've been able to follow him closely, he performed better than most expectations, keeping teams with mediocre (at best) personnel in the AFC playoff picture into the last month of the season.
This is a coach who has by most accounts overperformed the last two seasons, and the one time in his career he had a legitimate chance to put up an outstanding record, he did just that! Add to that the way he has come out of the gate in 2008, which is only his second real chance to put up a noteworthy record, and I see no reason for pessimism.
You say you're hesitant to call him a "great" coach, and I'm in full agreement with you there. Obviously he needs to at least win a playoff game before anyone even thinks about calling him a genius.
But do you see any reason to think he isn't every bit as good as former world champions Brian Billick and Jon Gruden? 'Cause I sure don't.