His jumper isn't slow, it's just a little funny looking. At 6'10", he's extremely tall for a 3. As a power forward, he has an exceptional handle and ability to break down his man. He sets up his jumper with his dribbling very well, and should be as good as Paul Pierce or anybody else in the league operating from the high post, within a couple years. That's where he's very special.
With a 36" max vert while still recovering from his acl injury, he should be able to add height to his jumper as needed.
Guys who are 6'10" with long arms, good handle, herky-jerky moves, and smooth shooting touch are very rare and very valuable.
I think he'll be a handful for Pierce in practice even as a rookie, and even more so as he fills out his frame.
At 230 pounds, he'll be able to handle all sorts of contact as he works to get his shot off. He also shoots 80% from the line already, so he'll rack up the points just drawing fouls with his herky-jerky moves and pump-fakes.
He has a killer instinct and a lot to prove.
Durant light, Carmelo light, call it what you will, but give Miller the green light and let him work in isolation, and he will be a great scorer by the time his rookie contract comes up for extension.