The difference between Mayo and Beal isn't athleticism. It's that Beal is an 'elite' shooter and Mayo is just a 'good' shooter. You can teach ballhandling, you can't teach elite shooting ability.
Beal compares pretty favorably to Ray Allen. He came into the league at 19 and put up comparable stats to rookie Ray Allen who was 21.
http://www.basketball-reference.com/players/b/bealbr01.html
http://www.basketball-reference.com/players/a/allenra02.html
Allen was never particularly prolific at drawing fouls on a Kobe, Wade, Harden level. But he was an elite shooter, somebody who defenses had to run off the line all the time. Beal is only 20 and already developing that reputation as a guy you cannot leave on the three-point line. Putting it on the floor will develop naturally, ballhandling ability is one of the easiest traits to develop. Kevin Durant, Paul George, and many more all were terrible ballhandlers coming into the league and now are nearing elite status in creating off the dribble.
That metric for comparing Ray and Beal as rookies is a moot point because if we are talking who compares to who as a rookie, Mayo blows both Beal and Allen out of the water and we both know Mayo Nor Beal is no Ray Allen as of right now.
But back to your point, Ballhandling ability is one of the hardest traits to develop, at least in my opinion. It's one of those things that by the time you reach the NBA, I feel like in most cases that's what you are as far has your handle. The thing that improves most out of players once they hit the NBA is shooting accuracy. All the other improvements one sees out of Pros are due more to opportunity than actual improvement. Most players if you look at what they do best in the NBA, did the exact same thing the same way on the college level. The only thing that changes is opportunity.
I also can't call Beal an Elite shooter yet. He's literally 6 games into his sophomore season and his rookie year wasn't even that spectacular with respect to "Elite" shooting. Beal hasn't done enough to warrant an "Elite" shooter tag and I feel that there isn't a big difference between the two as of right now. I would love for Beal to start becoming better off the bounce, but I honestly don't know if it'll happen and that to me is when Beal will become not only an elite shooter, but an elite 2 guard. The difference right now between the two is Mayo's ability off the bounce, which isn't spectacular, but it is better than beal's. Everything else they are about the same, with OJ just being a tad bit better. But Beal is 20 years of age so in theory, he shouldn't be a finished product, but we'll see.
