How do you (and I have my own answer here that I think is the right one, but I just want to hear you expand on that) account for players who appear to see passing options and gaps in coverage no one else does, but then look like complete dopes when they have to cover someone?
Great question. There are several forms and components of defensive play.
Mental/anticipatory defense. Like you said, identifying lanes and openings (again, in reverse). These guys tend to be very positionally sound..."in the right spot". Think Gretzky, Crosby, etc.
There's "compete" defense. Guys that work their way to the puck all the time. If they don't have the mental game, these players are often "chasing the puck" around...think about the working wingers that you've seen over the years...Tyler Kennedy, Andrew Cogliano, whoever...
There's physical defense...guys that get it done by disabling folks...Radko Gudas and the like.
But then there's also the techniques of technical defense. How to really execute a check, basically. Body position and timing, stick position and timing, etc.
So, you can be very smart and choose not to compete defensively and not accomplish anything. You can be very smart and not really understand how to technically play defense and they'll be less effective...you can be smart, competitive, AND know what you're doing, but you could be Rocco Grimaldi's size and not be able to execute physically. You can easily see the patterns here, no doubt.
One of the better examples of what you're talking about is Mario Lemieux. Lemieux didn't play much defense...but when he had to, he was the best at it.
We see this adaptability with others...Joe Thornton internationally is not asked to play a top line role because of Canada's center depth. He comes in as a fourth liner and he checks...and he's damn good at it because he has adaptable, transferable skills.