First, I have no idea what kind of pro player Tebow will make. He's obviously one of the best and most accomplished college players in recent history. But what would he gain exactly by coming back, comparing to what he might gain (or lose) by leaving early?
By coming back he could win the Heisman (he has one), a National Championship (he has two), or he could just "enjoy the college experience." Though I imagine if Tebow says that he'll have something different in mind than Matt Leinart did when he offered that reason. (Leinart took a single music class his senior year, but presumably kept an otherwise busy schedule.) Tebow also might be able to improve his draft stock by improving his footwork and decision-making skills.
But why not just work on those things in the pros? It's unlikely that Tebow would be able to suddenly vault into being a first or even second rounder next year, particularly since Florida is not exactly going to change the gameplan. And if he wants to work on things like his footwork, reads, and learning the contours of an NFL offense, why not do it in the NFL? Many of the current starting quarterbacks all had significant apprentice time on a roster, and I think we all expect Tebow to work rather diligently at improving once he is there. And he does have some physical tools. I mean, he could hire some quarterback gurus and tutors for the offseason (he would be doing that anyway), but once in the NFL he would be able to work for a couple of years on nothing but the things that might make him a successful NFL QB.
Another factor is his mediocre draft grade. Although at first blush this seems like a disadvantage, my guess is that that whoever actually does draft him will do so because they see potential in him. They probably too will know a thing or two about developing quarterbacks. (Maybe Urban's good buddy Bill Belichick will take a flyer on the kid? Worked for Matt Cassel.) So he might land in a great situation where he'd be able to work on his game and develop.
The biggest possible disadvantage I'd see for Tebow is that, if he is only projected to be a fourth through sixth rounder this year, by leaving he might lose an opportunity to leap into the second or third round next year where he'd make significantly more money. But something tells me that is not the sort of thing driving him. And in any event, that's a speculative reason; his draft grade could also remain the same.
Finally, one factor plays a part with Tebow that normally wouldn't with other quarterbacks, but often does with runningbacks: hits and shelf life. NFL runningbacks have a limited shelf-life, before their skills and ability quickly diminish. The common wisdom is that this is a factor of the hits and punishment their bodies take. Tebow, unlike most future pro quarterbacks, takes many hits like a runningback; indeed, he often delivers them. It's a crude measure, but compare the total carries by Tebow to Texas Tech's quarterback, Graham Harrell, over the last two seasons. (In college "carries" includes sacks.) In 2007 and 2008, Harrell had 79 total "carries." Tebow? 386. Adding another 200 carries, sacks, and other hits might just start to wear on his body. And we all know NFL guys will be trying to hit him hard and often when he gets there, so body preservation might be reason alone.
So there it is. I'd bet that he comes back, but it's worth considering some reasons why he might not. The biggest is to get in the right situation with the right coaches. I mean, this afternoon we have Jake Delhomme playing against Kurt Warner, two guys who made their careers by beginning as backups and developing once they were in the league (or in the Arena league or NFL Europe). Leinart? He's on the bench.
Update: Well, Tebow is coming back.