The big knock on Fox is that he knew after 2008 that he wouldn't be extended, and then basically mailed in his final two seasons (as Jerry Richardson is a greedy bastard and refused to sign paychecks for a coach who wasn't an employee, so he just waited for Fox's contract to expire). His judgment in assistants is questionable; in Carolina, he allowed Mike McCoy (the current Denver offensive coordinator) to walk in favor of Jeff Davidson, who sucks at football.
He has a background as a hard-nosed, 4-3 defense guy, but I don't think that will spell doom for the Broncos. They were 4-3 in 2007, and in 2008, the party line was "we're experimenting with 3-4," which basically means "we're grasping at straws and trying to plug holes in this sinking ship." The secondary needs to be rebuilt, but the front seven are replacement-level at worst (excepting Dumervil, who's all-world).
It makes sense for the Broncos, really. They're going from a young, terribly unproven, brash, offensive-oriented head coach who was given the keys to the kingdom to a conservative, defensive, proven veteran coach who will have to answer to a general manager running the show (although we all know it'll be Elway). It might not work out, but from an organizational standpoint, it's a decent move. I'm interested to see how Fox and Tebow get along, though.