I can see Mario Hezonja being a star.

[Roy H.]

The lack of minutes / production scares me.

http://www.basketball-reference.com/euro/players/mario-hezonja-1.html

I see the upside, but what's his baseline?

I'm not worried about his minutes.  He's young, its Europe and his team doesn't want him leaving for the NBA but knows he will.  With his great shooting, its hard for me to see how he busts.  I'd say his floor is a high scoring 6th man who you wish would contribute more in other areas. 
 

It's not just the minutes, but the per-minute stats.  His scoring rate has been very mediocre in Europe, and most of his offense is from beyond the 3PT line.

I have no idea why.  It could just be a matter of coaching and his role.  If picking between him and Justise Winslow, though, I'd lean toward the more proven player.  (That would be a nice problem to have.)

[sahara]

Everyone hates the Celtics, I think this cocky talented kid would be a very nice player for us.

[Jferrari401]

I think Hezonja is going pretty high and we'd have to give up too much to get him. I would rather take Stanley Johnson and I wouldn't be surprised if he ends up better. Johnson seems like he has all the tools to be a great two way player. Plus wasn't he very highly rated going into his freshman year?