Lots of things? 75% of the time he handles himself very well defensively outside of the fact he isn't physical in the slot because of his size.
Yes he's prone to some lapses and bad plays but so is 99% of other Dmen in the league. His lapses happen more often then other guys keeping him from being an elite Dman. but nobody around her thinks he's elite. He's a high end #3 who can play above his role for short periods and do well in the role. He's not great defensively but he's not bad either and most the time he plays at a level above what replacement level would be.
I think that 75% is being pretty generous. I'd say it's more like 25% of the time he handles himself well defensively.
He has the occasional good play defensively that I think people like to point to as evidence that he's not that bad, but these are few and far between and they don't make up for all the other plays.
When he ends up in 1 on 1 coverage, which is a big part of playing defense, the majority of the time he just shadows his assignment around the ice. Basically just trying not to get danced around, but letting them skate with the puck and pass to whoever they want.
When he's in more of a zone coverage, he doesn't break up a lot of passes with his stick, and he's got a bad habit of puck watching and letting his assignment get wide open for passes. This happens somewhat often actually. He's not great at defending players in front of the net, but that's to be expected given his physical limitations. If he gets pressured deep in his zone, he also has a bad habit of rushing passes off the mark that end up in turnovers.
This kind of defensive coverage leads to sustained pressure, momentum swings against the Avs, building the confidence of the opposition, and the tiring out of Avalanche players. Not to mention high quality scoring chances, and the occasional goal against.
There are lots of situations that he is below average at in the defensive zone, but what helps negate some of the impact is that he and the coaches have found ways to get him out of the D zone with the puck. When he's on his game, he's very good at skating the puck out of the zone if he has a little time and space to start with.
He also benefits from having so few D zone starts as well. Out of the 240 defenseman that had 20 or more games in the NHL last season, there were only 8 that had a lower percentage of D zone starts.
Sergachev - 29.8
DeAngelo - 33.5
Franson - 34.2
Shattenkirk - 34.6
Krug - 34.8
Kronwall - 34.9
Hannifin - 36.9
Heed - 37.0
Butcher - 37.6
Barrie - 37.6
NHL Advanced Stats / Analytics | Hockey-Reference.com
With all of that said, he's an elite 5 on 5 PMD, that really found his PP game as well last season. This makes up a lot for his defensive shortcomings, but those shortcomings also negate some of his offense.
He's a high risk player that's one of the best in one area of the game, and not so good in the other. That's why he's so polarizing, because not everybody prefers a defenseman like that.