Tebow is entirely reliant on his defense keeping the game close and low scoring in order to win games. The problem is, if they get up against a decent offense that will put up 28+ points, Denver is screwed, they don't have any offensive firepower. You can't expect to win games consistently 14-10 (or, conversely, 45-42), some games your offense or defense will have off nights, and the other side of the ball HAS to pick up the slack. Denver just doesn't have an offense capable of that.
The fact that NFL talent is concentrated to the best of the best leaves less room for error than in college and HS, which is why Tebow will not succeed - he could flat out out-talent other teams at Florida. That won't fly in the NFL. It'll lead to a whole lot of mediocrity, at best - sometimes it'll work, sometimes it won't.
One thing you have to remember is that the top 1% of college players will enter into the NFL, many of those are cut for not being as good as existing players before the regular season even starts. Even the worst NFL player was still one of the best college players.
EDIT: It still boggles my mind how much credit Tebow took from amazing defensive performances. "Oh, Tebow wins another close game!" Should have been stated as "Defensive performance nearly ruined by another anemic offensive output."