Ah, moving the goalposts again, are we??
Note when I described and used +/- it was over a period of several years, which is what is needed to stabilize a variable stat like +/-. And of course you used Doughty's +/- this year, which is his first bad one after a stellar career where he has basically been plus after his rookie season. Maybe he has lost a step and is starting to slip; I don't know -- it will take longer to make a judgment on that. And yes, I'm sure that part of his issue is Jonathan Quick's deterioration this year, though their backup is 3rd in the league. But it MAY WELL BE just a 1-year aberration.
Similarly, you bring up good ol' Jeff Schultz and his one aberrant +/- year. Actually, he was a pretty good positional defender back then, and a good complement to Mike Green at the time, but positioning was all he had, and by a few years later the league had passed him by because of the increasing league emphasis on letting speed win out. It got to the point where Schultz couldn't compete.
As for Kuzy, HIS +/- this year might actually be fairly indicative of what he has been; a top-line center with a lot of talent, and talent around him, who has also been pretty inconsistent in his play AND has garnered a lot of his stats on the PP, which doesn't count in +/-, so he ends up basically an even player (I consider +6 over 60 games basically even, especially for a guy who plays as many minutes as he does.)