Level of competition, suspect shooting ability (still an issue), and his super skinny frame. Would he fill out? Could he play NBA basketball against grown men? Does he have a position in the NBA?
Most of those questions have been answered at this point. He's basically a 6'11'' version of Nic Batum.
Giannis is one of like four or five guys I remember being excited about in the 2013 draft. I seem to recall liking Kelly Olynyk at the time, but I was hoping the Celts would take Schroder, Dieng, or even Plumlee. Giannis seemed to me like a likely bust -- another Anthony Randolph -- but I think even at the time I wanted Ainge to take him because we were entering rebuilding, so might as well go for the home run if nothing else.
Hindsight is 20/20, of course, and Kelly hasn't been a bad pick. Still, a number of guys taken after him would probably be better for the Celts right now.
Even Shabazz Muhammad ... I think I was torn on him. So highly touted heading into college, but would he just be another Nick Young, or worse? Jury is still out on that one.