There are so many positive things to say about Tatum, but probably the thing I like best about him is his composure and willingness to let the game come to him. It's uncommon for him to force plays that aren't there (bad drives/passes), and he doesn't usually force shots either. He's shown that he can hit big shots in the 4th quarter and make FTs under pressure. I strongly believe that he's learning to play the "right way" - making the correct play at the right time and not worrying about stuffing the stat sheet.
He's also dispelled many of the concerns about his game. There were concerns about his "average" athleticism, but he's shown that he can get to the rim with one dribble from the 3 point line with relative ease thanks to his burst (good by NBA standards, but definitely not elite) and length. There was a question mark about his shooting, but he's been lights out from 3. And as above, he's playing within himself and playing team-oriented ball, and isn't a ball-stopping isolation player that he was at times at Duke.
Add that to the fact that he's scoring incredibly efficiently (49/48/85), helping significantly on the boards, and playing solid defense by rookie standards (good awareness and team defense, his length helps him average almost 1 BPG and 1 SPG), and we have a future star on our hands.
I personally don't care too much about basic stats - PPG especially given that we have Irving, Horford, and Brown (and next year Hayward), so if it still hovers at around 13-15 PPG next season I won't be concerned. This kid is going to be a killer on offense and in transition, and he'll be a solid, versatile defender. I can't praise this kid enough.