it's funny, with most of the really bad free throw shooters i can remember -- shaq, dwight, etc. -- there was something distinctly wrong in their motion. DJ seems to be able to shoot badly despite doing everything right. i remember one analyst talking about how it was actually harder for big guys to shoot free throws because their hands are so large that it's hard for them to hold the ball right. let's see where he is next year.
DJ has pretty bad form. His feet are squared up with both feet on the line, he follows through with four fingers, follow through goes to the right, and he stares at the rim too much. His release point is nice tho. Watch the good free throw shooters. KD does this and I was watching Kyle Lowry do this but their feet are turned 10 degrees left, body aligned with feet and turned a little left, right foot on line left foot behind a little, follows through straight forward with their index finger down and they watch the flight of the ball. KD also does this shoulder shrug every single time which I would conjecture acts like mental trigger
Watch DJs misses. The ball will fall to either side of the rim a lot. Great shooters (free throw and otherwise) miss short or long never to the side. Just watch any good shooter. Most of the times they miss it's too short but it's always centered.
I think the problem with most bigs free throws is that they never developed good shooting habits. Vast majority of good shooters align their hip and arm to the rim not their feet to the rim. When you square your feet to the rim it forces you to squish your arm and elbow into the middle to line up your forearm to the rim. Which is really really tough if you have massive pecs like most bigs. DJ doesn't even bother lining up his arm and elbow to the rim. It's kinda off to the side which makes it harder for the ball to fall straight. If he turned his body a little sideways he would make more shots fall.
You can have decent free throw percentage with the weirdest form but it probably takes a shit ton more work and you'll never be a great shooter. Like Shawn Marion and Joakim Noah have the worst forms. they make a decent percentage but they probably had to work hard at it and they're definitely anomalies
I forget who it was but I was watching another guy with feet squared up but his body was turned slightly left and he makes a pretty good portion of his free throws