I think the two things that impress me most about each guy:
Avery Bradley
The thing that constantly amazes me about Bradley is his ability to force turnovers just from how uncomfortable he makes his defensive assignments. I've lost count of the amount of times I've seen Bradley press guys (either full or half court) and the offensive player has either:
a) Dribbled off his foot
b) Thrown a bad pass (either out of bounds or straight to a Celtic)
c) Stepped out of bounds
d) Forced up a bad shot
e) Pass up the ball early
He just seems to be so good at getting into an opposing players' head so well - it's like he actually makes opposing guards scared to bring the ball up. He seems to completely take away their confidence to the point where he can make a 10 year veteran PG look like a rookie out of high school, consciously thinking about every move they make and constantly looking over their shoulder every time up the court. You can just see that after 5-10 minutes of game time with AB pressing them, they just give up completely and change their entire style of play.
That's an unbelievably powerful thing for a defender to be able to do - to be able to make your defensive assignment completely lose confidence in their abilities and start altering their game purely because of what your defensive pressure. Few players have that type of defensive talent.
I feel like if you looked at the stats for the most turnovers forced (not just steals, but all turnovers) while being defended by a player, Bradley would be right up near the top of that list.
The most frustrating part of all is that as an opponent, you can't even hate him for what he does. He's not like one of those KG style defenders who gets in your head with cheap shots, trash talk and hard fouls. This kid is the ultimate gentleman - quiet, polite, humble, never gets involved in fights, rarely ever argues calls, never commits hard fouls. He's such a professional and such a nice guy that all you can do is respect him and be happy for him the entire time he's embarrassing you on public television.
It's kinda like watching Forrest Gump - you see this quiet, unlikely kid achieving things you would never believe right before your eyes, just through straight up hard work, and you just can't help but root for him.
Marcus Smart
The most impressive thing about Smart to me is his ability to somehow always get back in front of a player when he's beat.
Smart doesn't seem like the quickest player in the world laterally, and he plays pretty aggressive defense, so it's just natural that from time to time he's going to get beat off the dribble.
But I swear every single time that happens I sit there thinking "ahh, there you go - Smart finally made a defensive mistake", only to find that within half a second to a second the defender has stopped and backed off because Smart is back in front of him again!
I don't know how he does it - I'm no pro (I only play casual basketball with friends) but my greatest strength as a player has always been my defense, and from my own playing expriences there are very few things in basketball that are harder then catching up to a player who has just beat you off the dribble while you're backpedaling on defense.
I mean, surely by that point in time offensive player is thinking "I left him in my dust, easy layup coming my way!!" only to look up a half second later and see him right back in your face again!
I don't think I've ever seen a defensive player who can do that as consistently as Smart does. Almost every single time he gets beat, he ends up back in front of his defender again. As somebody said, it's like he's a Jedi and he's used some trick to slow the defender in time. Either that or he's just a LOT quicker off the mark than he looks.