4. Size is no longer an advantage
In college, Tim Tebow uses his size to steamroll opposing D-lines in goal line and short yardage situations. However, in the NFL, it will be much harder for Tebow to impose his will on bigger, stronger defensive players. Additionally, there will no doubt be a big bulls-eye on Tebow's back after winning the Heisman Trophy, and many defensive players will be looking to take big shots at Tebow. Because of this, injuries might become an issue for Tebow early on as well, as he doesn't seem to know how to slide when running.
3. There won't always be open receivers all over the field
Dan Mullen certainly knows how to draw up plays that usually leave one of Florida's speedy receivers wide open and in positions to make Tebow look good. In the pros though, Dan Mullen won't be drawing up plays for Tebow. Tebow will get used to whatever offense is run, but the fact that there are more experienced DBs in the NFL than there are in college means throwing the ball is going to be much harder for Tebow.
2. Overall game and player speed
College defensive players certainly are by no means slow (especially in the SEC), but when compared to pro defensive players, there is a noticeable difference. In college, Tebow uses his speed to outrun defensive players and make something out of nothing on certain plays. On occasion, Tebow even uses his size and speed to run over defenders in the open field or on the goal line. If he tries this in the NFL though, he will quickly discover that his legs can no longer be the weapon they are now. Tebow's slower release could also become an issue in the pros. The faster defensive linemen will have more time to get to Tebow and disrupt his throws.
1. Mediocre offensive talent in key positions
At Florida, Tebow has a top-notch receiving corps, decent running backs, and a pretty solid O-line. More than likely, Tebow will go somewhere in the top 10 picks of the draft, meaning that he will go to a team that needs a lot of help. These teams usually only have mediocre offenses—something Tebow will not be used to.
He might not be the next Rex Grossman or Danny Wuerffel, but don't expect him to be the next Tom Brady or Peyton Manning, either.