There is likely to be a strong correlation between a defender's visible positive impact and his overall net impact. In other words, we should expect better defenders to tend to have both more positive plays and fewer negative plays. This is because of the symmetric nature of the distribution of human performance.
This pretty squarely summarizes the Brian Russell debate. Russell was supposedly contributing in the shadows, but whenever the play neared him, was weakened like Kryptonite by the panning camera. For two years, some would insist that though Russell looked like a muggle on plays he was involved in, he somehow tied together the Seahawks secondary like a pissed-on rug. Well, I didn't buy it and neither apparently did the league.
Brian Burke constructs a very solid case for WPA+ as a measure of individual defensive performance. It's one of the first non-traditional defensive stats I find interesting -- even exciting. I tread carefully with statistics. There's more garbage than good. However, at their best, statistics provide a scope and objectivity scouting is incapable of. This might be one. I think this is one. I am excited this could be a quality statistic that actually measures defensive performance. !!!