John Isner has the game to go deep at the US Open yet I am shocked he only reached a slam quarterfinal once in his career. Isner yes he got a big serve but he volleys well and he is good at the net. Yet John's return of serve is poor why? I have seen Isner matches and it seems like he tries too hard to hit a big shot and usually the ball goes out. If you were coaching Isner how would you help him improve his return of serve?
As President said, his reflexes are not that great. Not sure about the NBA comparison, I don't watch basketball, but can't it be that they look a bit faster and explosive than they really are because most players are tall? Obviously, they are better athletes than Isner, but still.
I don't think there's ever been a guy above 2 meter with a great or even decent return game. Delpo is the tallest guy I can think of with a relatively good return game.
As for Isner, it's not just about the reflexes on the return, it's as much about him not being that good a tennis player in general. Sure, his forehand is quite good, but as a returner, he's behind in the point from the start and will have a hard time getting in a position to hit a good forehand. His forehand is much, much better on his own service games, because the serve will set easy forehands time and time again.
So essentially, he's
a) not fast enough on the return in and of itself and his wingspan isn't enough to help him out with that and
b) he's not a good enough rallier/tennis player to handle the effect of being behind in a point as he usually will be, when he's not serving
That's at least how I see it