Perhaps I'm naïve, but I think Mario is doing fantastic.
He has a young team, with injury prone veterans. He is essentially a minor-league coach, or a junior coach. Likeable, someone you play your heart out for, and most importantly for a team that relies so heavily on youth (both for opening day roster and for ALL of the call-ups) MAKING A BUNCH OF YOUNG KIDS OVERPERFORM.
Sure, the veterans haven't been incredible, but his work with the kids (in particular with a young defense mostly six to ten years away from their prime but playing with relatively advanced defensive positioning considering their age) has been excellent.
He isn't an X's and O's coach, that's what the assistants do on this team. But he's great with the media, great with the personality management of a young emerging team and, perhaps most importantly to the business side of the game, a great ambassador for the club.
The great thing about professional sports is that the extensive coaching staff means the head coach is able to serve different functions (namely allowing them to focus on the personalities rather than systems). And as a personality manager, a figurehead, and an ambassador he is suiting these functions well.
His in game management, and line combinations, seem fine to me. He is adopting what I call (as I'm from Peterborough so let my biases be known) a "Dick Todd" system, where the lines are nearly all able to be rolled in order (they aren't) and have quality all the way through. His concentration on chemistry, not skill, deserves praise. As does his deflection of attention away from Spezza during post-game conversations. As well as his decision to ride Anderson (a work horse) to protect his young rising star from fatigue (keep in mind the number of minutes Lehner has seen thus far in his career at all levels). This allows a fresh Lehner for the playoffs or stretch run, with an established veteran #1 if he falters.
But I think the thing I like the most about his coaching, despite his being faulted by some on these boards for not employing a more rigid structure, is that he hasn't coached the creativity out of the kids just yet. Sure we trap with the best of em, and grind away at better opponents, but while the kids are learning the rigid structure of the NHL he is allowing them to make boom-or-bust mistakes. He's letting them play the best they can play RIGHT NOW, while teaching them systematic hockey that, once established NHLers. is the best system for the NHL. It's that room for growth, ultimately, that I think will pay off.