Quincy Miller never had the reputation as a particularly energetic defender, and with his knee possibly not going to make it back to 100% strength, he's going to really struggle guarding NBA 3s. He doesn't have the lateral quickness to stay in front of them. Being 6'10" obviously isn't ideal for lateral quickness against 6'7"-6'8" wings anyway.
He's also not particularly quick in general. He might get some minutes at 3 over the next few years as a scorer off the bench, but his lack of lateral quickness might limit his minutes.
I think stretch four is his natural position at this point, and I don't think he'll be very effective defensively for another couple years. The good news is he's already up to 220 pounds, and can get up to 235 in the next couple years.
He'll also have to reinvent himself as a power forward, and he doesn't have a particularly physical mentality.
Risky pick, as much as I like his offensive game.
His agility times from the combine will be released in the next couple days, and we'll have some better informed idea of his potential defense at the 3.