I think the only thing stopping Joel from being the 1st pick is past history with Bowie/MJ and Oden/Durant. No GM wants to make that mistake again especially in a loaded draft with multiple potential allstar prospects. He has all of the tools to become the next great center in the NBA, but how fast will he be able to pick up the NBA game and fast pace? And how much will he improve in terms of his toughness overall on both sides of the ball to avoid getting pushed around and beaten inside?
Also, does he have any previous injuries? That could be the other thing that holds him back from being the 1st pick, or a top 3 pick.