I think an important aspect for a coach isn't so much the overall success of their team but how their players react them. Like every cycle; DC, OC, HC get hired from top tier franchises and some of considered the gurus or architects of those teams, get a HC job and sometimes they pan out and a lot of times they don't. Think Adam Gase or Nathanial Hackett, but when they left their teams to become HC ain't a lot of their former players (that I can think of at least) saying much or hearing a bunch of stories of how their team or that unit they coached mad respected them, or other colorful stories. Is that required? No, but it's interesting.
When I hear about Fraley or McCown and how those players from those team love and respect them, like they're legitimate mentors and motivators, those are the ones I get excited about. Yes having playcalling experience is nice and is a real concern, but if I know a coach is gonna get every ounce out of a player, make them bust their ass and unite them as a unit and not just a collection of players following a gameplan, then that's what I want.