I think the entire Avs management team underestimated the impact of losing Stastny. Duchene and MacKinnon are simply not ready to take on the defensive responsibilities that SOS took on, and have not adjusted their play to be able to do so. I'd say Roy's greatest weakness as a coach is that he has a really long leash for his personal favourites. I get the feeling that he thinks he is a miracle worker and has made NHLers out of AHLers in Holden, Guenin and Cliche, the result is that they get played way too much, in pressure situations that they should not be in.
They are spending way too much effort to try to cover up for their complete lack of talent defensively. Last year the Avs defensive scheme worked because no one really played that style, however, teams have adjusted and the Avs have not exactly upgraded their talent defensively. We end up getting hemmed in the defensive zone for long periods of time, and with terrible recognition from the defensive players.
In the offensive zone the Avs are employing a grinding style of play, which does not mesh well with the players that they have. They should be constantly moving and forcing the defense to give them space. Instead they are taking it behind the net, and cycling it in the corners and then giving the puck up. No one gets open for shots, no one is moving, it's very easy to stop.
Long story short, it's a mixture of a severe lack of talent defensively, the offensive really under performing, and Roy not adjusting fast enough to the adjustments of other teams. I think he is a good coach, but he is still learning to coach at the NHL level. It's a growing process for him and the players.