Saying what does Mayfield have that Manziel didn't is a crazy question imo, because he has EXACTLY what Manziel didn't. Lets remember, Manziel didn't play for awhile and suck. He came into the NFL, partied, did drugs, didn't study the playbook, took it all for granted, and when the team was forced to play him a few times, he basically played pick up ball.
Mayfield is the exact opposite. He is a warrior and a leader. He will learn the playbook and work hard. I do hear the kid likes to have fun, but not in lieu of football like Manziel, who was entitled as shit.
So to say Mayfield = Manziel is not fair. I think a better way to put it is Mayfield could be what people hoped Manziel was going to be.