Tebow was "college accurate". To the point where he had a better overall passer rating than Peyton or Eli Manning, Tom Brady, Aaron Rodgers, or just about any other top NFL QB that you can think of.
I parsed the stats once comparing Tebow and Peyton, and the overall stats weren't even close. Peyton had more yards, but he had many more attempts and completions. In Yards per attempt and yards per completion, Tebow crushed him. He also crushed him in TD % and INT% iirc.
All that said, his mechanics were severely flawed and while he was "college accurate", the general belief was that he'd have major problems in the NFL due to the smaller/tighter passing windows. Didn't help that his delivery motion was elongated and slower than most, which would allow defenders to get a jump on his passes.
(This isn't a complete list of all the problems, more of a general highlight type of thing).
So, Tebow went to work changing his mechanics and it's been a very bumpy road since then. In Denver, at least during the first 3 quarters of games, it looked like Tim was thinking about mechanics while he was trying to throw the ball. Not sure if you've played golf before, but doing this kind of thing usually leads to disaster. When you are in the middle of a play (or swing), you shouldn't be thinking "left foot here, right foot there, ball here, hands there, bring it back, rotate, GO, follow through."
Later in games, generally in the 4th Q, Denver usually found themselves behind and went to a Spread formation offense with Tebow in the gun. He would revert back to his college style and to the amazement of many, he was pretty successful with it. Still not "NFL accurate" and able to hit tight windows on command, but he could get the ball out to receivers for the most part.
In short, at those times, he stopped "thinking" and just started playing.
This brings up what could be an interesting discussion, vis a vis, could Tebow have been successful in the long term had he never changed his mechanics. But I'll pass on that one for now unless others want to get into it.
Given that he has tinkered with his mechanics, the question at this point is whether the latest work that he put in during the offseason in AZ has fixed things. He not only worked on his release, but this time, they spent a lot of time working on his footwork as well.