As a big fan of USC, I always have to be careful rating prospects from there; it would be easy to gloss over their flaws if I wasn't careful. That said, there is very little wrong with Barkley's game. His footwork is excellent. He has a nice, tight throwing motion. He hits his target almost every time, though a bit less so on the deep balls. He is careful with the ball, but still makes the big plays. He sees just about every eligible receiver. There is almost never a reason to scratch your head, and wonder why he did something.
He does have two amazing talents at receiver in Robert Woods and Marquise Lee, but they are just a Sophmore, and a Freshman, which showed in their play at times. Many times, Barkley would throw them a perfect pass, and they would drop it. Neither is amazingly fast, nor amazingly strong. They would consistently beat their guys, but rarely were they wide open.
Sometimes, a QB will come out of a big time school having seen almost no pressure, but that isn't the case with Barkley. The offensive line did not do a good job protecting Barkley most of the time, and he had to get the ball out in short order.
In his mock, Scott Wright claims Barkley isn't elite, but I completely disagree. Barkley is a better prospect than any QB in the last several drafts. He is just unlucky enough to have Luck in the same draft.
There are legitimate reasons an individual could decide they would take Barkley before Luck. Luck's footwork is not that good, though it isn't horrible. Luck hasn't quite lit the world on fire like you might expect. Barkley has often been more spectacular. He has done more with less, and a year less in the program, than Luck has. At this point, I'm probably 60-40 on taking Luck before Barkley, but it isn't crazy to go the other way. Luck is a legitimate first overall prospect, but so is Barkley. My opinion of Barkley actually went up after scouting him, as opposed to just watching.
Barkley is by far the best QB USC has had since I started watching them (admittedly, that only goes back to the Leinart days). The size of that gap is as large as the difference between a Peyton Manning, and a Jon Kitna (who, don't forget, could put up big numbers, but is a long time journeyman for a reason.) He is what made USC great this season. Obviously, I don't want Barkley to go pro, since he won't end up on one of my teams (that would require an epic fall)(go niners!), but he should. There is nothing left for Barkley to prove at the college level.