In the NFL you need a pure passer to win on a consistent basis. Players like Vick and Randal Cunningham can show signs of dominance and win a few games, but it comes down to the fact you have to pass the ball and have a quick release.
For Tebow and all QBs that enter the league it comes down to this, is his hand eye coordination able to keep up with the NFL speed? This comes down to how fast he can release the ball and how fast his mind can process what happening to release the ball.
Thats why every year we see examples of QBs that have all the natural gifts, yet fail. Tom Brady on the other hand will not wow you as a physical specimen, but plays year in and out at high level. Tom Brady's real gift is his hand eye coordination, he maximizes his athletic gifts with his ability to see and react quicker than most QBs.
I think Payton Manning knows he does not have the natural ability to read and react as fast as some of the better QBs in the league, but he makes up for this in his film study. I think a lot of times Manning knows what he is going to do at the snap of the ball where as Brady and BF can process things in a second or two. Both have been sucessful.
Tebow is no different from any other QB entering the league, he just has proven to be a runner. He has the same challenges as all the other QBs, can his mind process the game quick enough?