Greg Russell already pointed out the double standard. Jimmy Johnson builds up such an amazing dynasty that even Barry Switzer manages to succeed and it proves Jimmy Johnson is a genius. Tony Dungy builds up such an amazing dynasty that even Jim Caldwell manages to succeed and it proves that Tony Dungy was easily replaceable.
As for John Fox... no way was he essentially a .500 career coach. He took over a 1-15 team and went 71-57 with them (55.5%), including 5-3 in the playoffs with two NFCCG appearances and one SB appearance, before ownership pulled the rug out from under him, gutted the team, and let him take the fall for a lame-duck franchise. He then took over a Denver Broncos squad drafting #2 overall and got them to 8-8 and the divisional round of the playoffs. All without Peyton Manning. Yes, Manning makes a difference, but again... Manning is not the only Hall of Fame QB in history. Tony Dungy's record with his HoF QB is better than Bill Belichick's with Brady, or Mike Shanahan's with Elway. Dungy's record *WITHOUT* his HoF QB is better than Belichick's without Brady or Shanahan's without Elway, too.
The fact that people feel it's necessary to add this "perspective" (hey, perhaps his record would have been worse if he hadn't played with one of the top 5 QBs of all time) to qualify Dungy's place in history and not, say, Belichick's or Walsh's is exactly what I'm talking about when I talk about how people hate on Tony Dungy.
I wouldn't call what Indy was a dynasty; one title over a decade, despite all of those 12+ win seasons, is not a dynasty. But I have been consistent in this regard, and that is another coach coming in and winning with a team that had already won (like Switzer did with Dallas) is more of an indication of the great talent said team had than it was of the previous coach's greatness. In the case of Dungy, how much control did he have over player personnel in Indy? Caldwell being able to step in and win 14 games in his first season said a lot about a) Peyton Manning's greatness, and b) Bill Polian.
I agree that Fox was better than his record was in Carolina, and that is my point: you can't always look at a record and say, "That is the story." Often times, other factors play a huge part in a good coach's record not being as good, or vice versa. Hell, most think Wade Phillips is a great defensive coach, but not that great of a head coach, yet he still went 34-22 in Dallas, which was considered disappointing despite being, record wise, their best coach in the last 15 years. That is what I meant when I said I was giving perspective on Dungy (as I think he was a very good coach, but not great).