A lot of mainstream hockey journalists are lazy. They remember seeing a bad stat like 80 kajillion goals against and assume the defense must have sucked. Other lazy 'journalists' and bloggers parrot this. Average (and below average) fans read it and assume it to be gospel.
Here is a somewhat definitive look at how the Islanders in 2013 stacked up in some key metrics.
Even Strength Shots per 60 Minutes
Islanders 10 - 31.82 NHL Average - 30.87
The Islanders were a top 10 team in terms of generating offense at even strength. The addition of more depth at forward should help maintain this.
Even Strength Goals per 60 Minutes
Islanders 18 - 2.17
NHL Average - 2.24
While they did a good job of getting shots on net, they were just below average of getting those shots past the goaltender.
Power Play Shots per 60 Minutes
Islanders 16 - 52.54
NHL Average 52.29
Power Play Goals per 60 Minutes
Islanders 13 - 6.44
NHL Average 6.25
All in all, the Islanders were an NHL Average club when it came to offensive production. Lots of room to improve, but not the weakest link in the chain.
Even Strength Shots Against per 60 Minutes
NHL Average 30.87
Islanders 19 - 31.32
The Islanders need to improve on their shots against. One could argue the removal of top flight defense generator Andrew MacDonald was a huge step in that direction.
Goals Against per 60 Minutes
NHL Average 2.24
And here's where it gets ugly. Goaltending at even strength was a glaring weakness.
Penalty Kill Shots Allowed per 60 Minutes
Islanders 10 - 49.88
NHL Average 52.59
When it came to killing penalties, the Islanders were among the better teams in the league in limiting the opponents' chances. Of course, when they did shoot the puck, bad things happened...
Penalty Kill Goals Allowed per 60 Minutes
NHL Average 6.25
That's ugly. It doesn't matter how well you suppress shots (unless you eliminate them all) when your goaltenders allow them in at that rate.
It all started and ended for the Islanders with horrid goaltending behind a slightly below average defense and average offense.