Allow me to be philosophical for a sec. I'm leery of evaluating a prospect this way. "If he just gets better at x and y he could make a real impact". Or, "we haven't seen him do it yet, but the potential is there". In my experience, some things can be improved (like conditioning), but what you see is mostly what you get. Many of the most important things are innate. Take this description from DraftExpress:
"Shows tantalizing talent and an incredibly high skill-level for a 7-footer. Has great size, a nice frame, long arms and solid mobility for his position. Not an athletic freak, but very coordinated. Fluid, yet not overly explosive, particularly around the rim. Amazing perimeter shooter who can create his own shot with ease, but still has a ways to go in terms of learning how to play winning basketball. A mediocre rebounder and defender who is not all that efficient offensively. Still has considerable upside to continue to develop."
That sounds enough like Bender to me. It's actually Andrea Bargnani. So what's the difference between Bargnani and Nowitzki or Bender and Nowitzki? I don't know, but it's obvious if you watch a couple hours of tape. You don't watch teenage Dirk and think, "wow, he has a long ways to go". You don't look at 18 year old Bird and think, "if he was just a little stronger" (lol) -- he's already putting up 30 and 15 every game. When I watch Bender I think, "backup energy guy?"
It's not infallible and just an opinion. e.g. I didn't think Towns would be a first tier center in the league. I thought solid starter in a couple years. It looks like I was wrong. We didn't see everything he had to offer at KY.