The video that was linked to above nailed it. He does a lot of the little things very well on defense -- knowing when to switch/help/recover, going after defensive rebounds, staying in front, not biting on pump fakes, etc. Of course he's got a learning curve, but it's amazing how mature he looks on the defensive end. And of course, just being intelligent and fairly athletic with super long arms can get you really far as a defender in the NBA.
I felt like Avery Bradley was an excellent on-ball defender, but wasn't a great team defender (in part because he was a 6'2" shooting guard). Tatum can be the exact opposite: Not at all sexy, but exactly the kind of switch-everything, team defender that you need given the way the game's played nowadays.
Your eyes aren't fooling you: this kid is legit.