Denver was 1-4 with Kyle Orton. Kyle Orton is a back-up QB in the league right now and may not even be on a roster next year. Tebow comes in and gives the team an initial bump against awful teams and wins 7 of the first 8 games, before losing the last 3 and only making the playoffs as a result of some weird tiebreaker i.e. win percentage of common games (Denver had 5 wins, Oakland and SD only had 4). Some of that was lucky scheduling, like Denver playing Chicago after Cutler went down (unlike SD who lost to Chicago in Cutler's last game of the year). Denver gets rid of Tebow and ends up 5 games better with basically the same offense aside from Peyton for Tebow. Tebow goes to NY and not only can't beat out Sanchez, he can't beat out Greg McElroy as the main back-up. That really tells you what you need to know.
Also, I would like to point out that quite a few bad QB's have started out with good success who haven't been good QB's. I mean Rex Grossman started out 17-6, then went 2-1 in the playoffs (making the Superbowl) to start his career. Is anyone here saying Grossman is a great QB? Heck after the Superbowl run there was talk of Grossman not even starting to start the next year (he did, but it was discussed). Early success or early failure doesn't mean much, it is all about the skills shown and presented. Tebow just doesn't have them or some team would have given him the opportunity.