Two important factors at play here, imo: Maturity and Mind Set.
It has been discussed in depth here that Kreider possesses a ton of natural ability, an amazingly hard accurate shot, speed, size, etc., etc.
This is just my opinion, but the one thing he lacks now is a mindset that focuses on consistent puck pursuit.
Hagelin is effective because he uses his speed to pursue the puck, relentlessly pressure the opposition, and thus enhance every skill he has. When Kreider does this he becomes instantly dangerous...but that is the thing. He is not consistent in this area. If Kreider was harder on the puck his inexperience would take a back seat to the pressure he could put on opponents. I think this is the major reason he was sent back to Connecticut.
Admittedly, I'm a little concerned. A tape session can make things pretty clear. I have no first hand knowledge, but this has to have been pointed out to him. Repeatedly.