I have no problem taking it slow with Zeller. Although he may never break out as an all-star, I think he's too smart and skilled to ever be a bust, so we don't have to worry about that. Part of the reason is I don't think he has that aggressive, takeover gene that is in the DNA of stars. I think he likes to fit in, do what it takes to win, but I don't ever see him stepping on toes, demanding the ball to punish some little scrub guarding him. It seems he'll take advantage when he can, and fill in the rest of the time.
As for what I'm seeing, he gets knocked from his position a lot, so its a matter of building more lower body/core strength. His arms and shoulders look surprisingly big, so he may be focusing too much on upper body stuff. You can get more power with your legs and learning how to use leverage, which is something that can be built and taught. Once he gets some heavier britches, he'll be able to box out and rebound better, post up closer to the basket, and be more stout on defense.
On offense, he needs to stop hesitating at the top of the key and let the ball fly. He can hit that shot, he just needs to take it in rhythm instead of thinking about it and coming up short. I do like his touch around the basket, so we should start using some back screens around the mid post/short corner so he can get closer to those short range sweet spots that he can take advantage of regularly with his size and quickness. But its all a matter of confidence and experience with him, and I have no worries about him getting there.