Pros...
Good NBA body, and he is long
Flashes some nice passing skills, and dribbling skills. (But not real often)
Good Attitude/Good guy
Good defender
Cons...
Lacks elite athleticism
Timid
Shot looks weird
I still think he has a chance to be an above average starter, but I don't see a star, I also don't rule it out because he is so young and he does have enough physical tools to become something. I'm not gonna give up on him yet.
Honestly though, if we could redo the draft, I'd rather take Zach Lavine. His future is still up in the air, but I'd say he has a better chance at becoming a star. I don't think he is just a dunker. He can shoot it pretty well already, and has nice handles. Also, isn't afraid. If he works hard, the sky is the limit for him.
Over the next ten years or so, we could see different phases of who is the better point guard from this draft. Zach Lavine could become an early star, but never become truly great because he relies on his athleticism too much. While Exum could take a long time to develop, but eventually become the better player, because he becomes a smarter player.