See this is something I'm not sure why fans don't give him more flack for. I've mentioned it a few times about how it's uber aggressive sending in at least two forecheckers deep, and sometimes the third in the zone as well, and that it's not sustainable, but let me just elaborate a bit more.
It's basically designed to force turnovers, or force the defense to panic which will lead to bad passes or tunrnovers afterwards. Now this is great when it works, but it requires the forecheckers to be working 100% and have full energy. A young team might be able to get away with this energy wise, but they're also going to be making fundamental mistakes. An older team certainly can't keep up the pace required, but they'll be making less fundamental errors.
When it doesn't work, or when the players don't quite have their energy up, or have maybe a lapse in momentum, the Avs are very easy to play against. Teams move the puck quickly to each other, and have a wide open neutral zone to break in with. This is bad on it's own, but it puts tremendous amounts of pressure on the D (who are not very mobile in the Avs case) and leads to everyone scrambling to get in position and playing catchup with the puck carriers. This is just plain bad on many levels, and leads to goals against in many scenarios.
This is why the Avs can't hold leads, why they let teams back in games when they're ahead, and why when they're not quite on their game it snowballs in multiple goals against until the opposition sits back in the 3rd sitting on a lead.
The other problem with this is that it works the team so hard, it's no wonder this team fades towards the end of the season. They're gassed from constantly chasing the puck at full speed all year.
Not to mention a smart team will pick this system apart in the playoffs after closely examining it and exploiting it with quick, smart puck movement. It's also the reason Sacco has such an awful record against divisional opponent, and such a good one against the East, and in his first year where teams didn't know their style as well.