Truth be told, Luck is considered the best prospect ever because he has both the tangibles and the intangibles. He's a great leader, great person, his work ethic is phenomenal, he's incredibly smart, plus he can run and make all throws.
There are a lot of things with Luck that aren't certain. He plays in a run first offense, most of his reads are out of playaction. His offense is nowhere near close to McCarthy's, and he doesn't have even close to the responsabilities Rodgers has. He is projected to develop it in the future given his work ethic and smarts, but it's not a given. And it's one of the most important aspects of an NFL QB, the pre-snap and post-snap reads, and he's not doing it in college.
Another thing is that he doesn't have the same arm and accuracy that Rodgers has. He has enough to be able to make every throw in the NFL, but Rodgers is able to make every single throw with tight coverage and throwing out of his knees. They're projecting that his arm will increase ever further with weight training and conditioning, as Rodgers' has, but it's still projection.
Luck is the best prospect to come out of college and I agree with that. But he is still a prospect, not an NFL QB.