As for that "excellent" slider Chapman had when he started out in that 2011 season...
(23 years old) 50 IP, 3.60 ERA, 1.30 WHIP, 7.4 BB/9, 1.73 K/BB
Cabrera's numbers in Mississippi before being called up:
(22 years old) 33.2 IP, 3.21 ERA, 1.248 WHIP, 5.9 BB/9, 1.59 K/BB
The point is that his fastball is dominant enough to have hope that he has that type of upside. I've seen it mentioned several times that he's looked "bored" at times and that part of the lack of development of his secondary stuff could be attributed to the fact that he just hasn't needed to focus on it - I'd think the hope is that calling him up shows him just how dominant he could be if he just had an average second pitch.
HOw can he be bored, he's gotten lit up like a xmas tree in the minor leagues. He's not dominating guys and losing interest.
The problem is anyone can hit a 100 mph fastball when that is all you have to worry about. Until he develops control and an offspeed pitch, he isn't going to be successful. They thought of him developing both is intriguing and makes it exciting to thing about, but it is not an easy process.
Him not missing bats in the minor leagues like you would expect for something that throws that hard is a concern, but at 22 with a 100 mph heater, its fun to watch.
To add to that, he's not missed bats at all.