I heard things like...
1. He isn't strong enough to power through OLines in the NFL like he did in college.
2. Not a good pass rusher (though good at stopping the run).
3. If his quick 1st step is ineffective... he will be ineffective.
4. Lacks instinct and vision when he's in the backfield and routinely runs himself out of plays or misses at the point of attack.
5. Not as active at the line of scrimmage with his hands on quick drops as you'd like.
6. Relied on jumping snap counts in college for his "quick first step" and will face teams who can scheme, hard count, go on silent counts, etc. to neutralize that skill
7. Lacks sufficient upper-body strength to power through an offensive lineman who gets his hands on him and can get washed down the line by bigger opponents.
8. Was rotated heavily in college and does not have the stamina or experience to be an every-down lineman
9. Easy-going personality ; not "mean" enough to go toe-to-toe with NFL offensive linemen
All things I've heard and read from actual scouts, if you put stock into their opinions. McCoy screams mediocrity with everything I read and every bit of footage I watch him in.