The Broncos have by far the best roster outside of the QB position in the NFL, it's really not even close. Peyton was easily having the worst year of any QB i can remember in the last 5 or so years throwing 3 to 4 INTs per game, often times with no pressure (O-line weaknesses existst but is overblown). That tells you all you need to know about their roster.
Brock wouldn't be able to start for any other team in the league. He has a longer delivery than Byron Leftwich and frequently lets guys get too close when he's winding up to throw the ball, his ground game delivered nearly 200 yards against a top defense on a slippery surface, his receivers made big plays, which isn't surprising considering how talented they are. He plays for Gary Kubiak whose offense is insanely easy for the QB when the running game gets going, Kubiak made Jake Plummer a 90+ rating QB and that guy had a negative TD INT ratio his whole career. The last time that Cutler played for Kubiak, they were on pace to break the scoring record before 9 runningbacks went out on IR. When they lost the runninggame, Cutler started throwing picks, before that, I remember the nationwide argument on whether Rodgers was as good as Cutler. It's just a QB friendly offense, it sacrifices other areas to achieve this, but it works as long as you have a strong running game.
I would be slow to judge BO while he's in that system, with that kind of talent around him, as of now, he's a backup playing passable football, which is a huge upgrade over PM, but that's not saying much.
