Avenger, I agree that college success is basically irrelevant to the NFL. What I meant was, using "oh, he has had a new offense every year" isn't a full-on legitimate excuse for mediocre play.
Here, let's look at Football Outsiders rankings for the first 3 years of the careers of the guys you mentioned.
Eli: 32nd (of 42), 9th (of 46), and 13th (of 46).
Brees: Not enough passes in the season to be ranked in his rookie year. 2nd/3rd/4th are as follows: 21st (of 47), 40th (of 47), and 7th (of 42).
Bradford: 39th (of 46), 43rd (of 47), and 16th (of 39).
He did have his best year in 2012, obviously; but you want to know why I'm still not sold on Bradford? Even when he had statistically good games, he didn't really look good. I just looked at Pro Football Focus's grades for him for 2012, and that would seem to back my assertion. He had a negative grade for 8 of his 16 games. If you think he's going to turn into an elite QB because he showed improvement in his 3rd year, keep dreaming. PFF only goes back to 2008, when they started the site, so I can't check the first 3 years of Brees or Eli, unfortunately. However, I did just look at all the 2012 starting QBs on there to count how many games they had where they had a negative grade, and Bradford had 8. Half of his games had a negative grade for him; and PFF doesn't penalize QBs for bad O-lines, and they even remote interceptions that bounce off of the hands of WRs, etc. It's a fairly good indicator of how a QB actually played. Here are the other QBs with 8 or more games that had negative grades in the regular season last year:
Dalton, Palmer, Cutler, Freeman, and Sanchez also had 8. Weeden and Fitzpatrick had 9. Ponder was by far the worst, with 12.
Not exactly great company. We'll see what happens with Bradford this year, but you should definitely be in "if he doesn't show definite improvement this year over last year, I want some real competition in camp next year to have a chance at winning the job over him" mode, in my opinion.