Too many of the names being thrown around are, at least to me, life-long coordinators (Offense or Defense). They excel at that level, but don't have that spark that it takes to succeed at HC. I still like the old school step ladder for a successful head coach. Start out as an assistant at a College (preferably in a major conference) and work your way up to being a coordinator, then go to the Pros and do the same thing (work your way up). You then take a Head Coach position in College and succeed.
After that, you become Pro HC. It exposes you to everything you need.
It seems to me that there are several that get rushed too fast or are just not really HC material. If they had been given the opportunity of being a HC at College, their shortcomings probably would have been revealed (

Mike Singletary)
Retread Coordinators who never succeed as HC (

Norv Turner, and the rest) - GREAT Coordinators, but just not HC.