Dominated by the NFC West. A good read.
WPA is used, as pointed out, as a measure of 2011 impact. If you subscribe to the idea that the best players make the best plays, and that great play and fewer mistakes tend to go hand in hand, then you'll not find issue with this. There of course are exceptions, but greatness+hateness tend to surface those issues. For instance everyone knew that Shawne Merriman was below average at everything besides pass rushing.
If you subscribe to the idea that highlight-fixation clouds a much more complex tangle of great plays and similarly-impactful, hidden mistakes, then you'll likely find issue with any individual statistical measure outside of QB.
EPA is not used, as the purpose is accolades of 2011 accomplishment, not 2012 outlook on impact.