I think the coaches probably base it more on off the ice stuff, how they work in practice, what they're like in the locker room, if they're respected among their teammates, if they carry themselves like leaders. I agree, it's not something fans can really discuss much because they aren't on the inside.
I've said my piece about Gill plenty of times. He's obviously got his weaknesses. I think strictly defensively, he still plays smart and compensates for those. He's still their go-to PK defensemen. It came up in another thread, but the difference in the PK since he came over is striking. The team was at 79.6% before he made his Pens debut last year. They played at 84.9% the rest of the regular season, then at 87.1% in the playoffs, and they've been over 84% this season. I won't give him all the credit - his arrival also coincided with Scuderi breaking his finger, which opened the door for Orpik to assume a regular PK role, which he's done well with. But Gill's the guy playing big minutes there, along with Scuderi. I think his size is essential on the PK and it's something no one else on the team has. Once they have Gonchar and Whitney back and those guys are playing large minutes, they can more easily hide Gill on ES. I think a lot of the more frustrating moments from him actually come in the offensive zone, which isn't really his job anyway.