I stand by my Zac Dawson comment.
The way I see it there are three primary roles that a KPD has these days, gone are the days that you could just pay attention to your man.
1) Stopping a key forward. This is Hartley's biggest strength. He really does have a nice spoil on him and positions himself well when the midfield is doing their job and applying pressure. That said, panics at times which leads to him grabbing his opponents jumper (Glad people on here have stopped being delusional about this, remember when the umps were against us when he kept scragging Hawkins?)
2) Providing a link up option when switching the play from defence. He 'could' be an asset in this regard, as he has a nice long kick. But he never manages to get involved in this area. Perhaps it's a team direction, but I'm pretty sceptical. He had 1 kick on the weekend, and 4 possessions total vs the Swans. You put up numbers like that, youd better have Chris Langford-esque stopping prowess (which he doesn't).
3) Setting up the press. This is SUCH an important part of modern football, and our press on the weekend was garbage. Look at the way Lonergan/Henderson do this for the Cats, the intercept marking effective KPDs provide allow for quick, repeated f50 entries, putting pressure on the opposition and tiring them out. Hartley took 1 intercept mark on the weekend (it was great to see), but it's rarer than a Scott Lucas handball. He can't be considereda good KPD until contributes in this area.
So can he improve in these areas? Maybe. Fundamentally, I think his problem is footspeed. He is scared of leaving his man, or playing in front, in case he gets caught out and his man gets a cheapie. So he tails around his opponent, and grabs them by the jumper if the ball comes in to high and fast.