I think there are many times when they have been outshooting their opponents, getting better scoring chances, even scoring goals when they inexplicably go into a shell as you say and it cost them very important games. They almost tell themselves. "OK we have a one goal lead, if we just sit back and play defense we should pull this off" and the opposing team pelts our goaltender (yes oftentimes from the perimeter) relentlessly. Against mediocre teams like Columbus this might work, against the better teams in the league (which I think the Isles are as well) they are only asking for trouble. Eventually someone is going to miss an assignment, the goaltender will be screened, or there will be an odd man rush and it's game over. They need to eliminate this bewildering part of their game and learn to play to win for me to have confidence in this team. This really is why so many fans and the media lament the game plan the Islanders use. It's almost not sport anymore. It's too calculated and risk adverse.
I don't think it's the roster. Again, I think it's how they approach the game. Too afraid to take chances and open themselves up to defensive risks at the right time even when they are in the lead. There's no excuse for this line-up to be one of the lowest scoring teams in the league year after year. That's all a mirage caused but the risk adverse game plan they utilize. If you can accept an Eastern Conference Final appearance as being this team's Stanley Cup final then accept the way they play the game. Sure, there are 32 teams in the league and only 1 can with the Cup each year but to watch this team lose year after year for the same reason and not do anything about it edges on insanity. They need to understand that sometimes the best defense is a good offense. The time to change their mindset is now, not during a playoff run.