It's too hard to project who is going to be a good NFL quarterback; conversely, it's easy to predict who isn't. I remember sitting back and watching the 2007 Draft and, as a Broncos fan, just laughing my ass off when Jamarcus Russell was picked by Oakland.
If you think back over the last 20 years, the best QB's have, for the most part, been completely out of left field:Tom Brady, Kurt Warner, Brett Favre. Drew Brees was taken in the second round but who really thought that a 6'0" tall quarterback with questionable arm strength was going to do what he's done? How about Mark Brunnel, he had a nice run for five years and most didn't see that happening. Even Tony Romo--who isn't great but is good enough--was a virtual unknown.
It's still true that every once in a while a highly regarded QB lives up to his potential: both Mannings, Rivers (to some degree), McNabb and Matt Ryan--at this point--come to mind.
But there sure have been a lot of busts. I would never want to be an NFL GM in need of a quarterback on draft day.