The "IT" factor is BS. The "IT" factor is not a measurable.
Do you realize that prior to his foot injury, Schaub had a 120 rating against the blitz? How many times prior to the 2011 season did Schaub show up at the end of a game just to be let down by his defense or his kicker? After his foot injury, he was a totally different guy. It took a while for defenses to realize it but once they did, he was toast.
Your "IT" factor is just looking at a successful guy and then saying he's got the "IT" factor because he's successful. It's a classic tautology.
Coming out of college, Tom Brady didn't have the "IT" factor; he couldn't even clearly win the QB job at his own college. And his first couple of years in the league, he wasn't that good. And if it hadn't been for a little known rule that's no longer on the books, he wouldn't have won that first SB and he probably would have had his hands full beating out Bledsoe for the Patriots job.
Oh... but now... after a successful career and with 20-20 hindsight, Tom Brady has "IT".