Show how many stddevs better than expected I perform just because card X is in the game.
This would be more useful to me than some of the current stats. If I perform -2.3 stddevs better than expected when Pirate Ship is in the game, that tells me that I am playing that particular card ineffectively -- whether that means I'm buying it when I shouldn't, not when I should, at the wrong time, not defending well against it, *whatever.* Then I can do some deep thinking about my strategies surrounding that card.
George Locke commented
"My problem with the popular buys page is that there is already too much stuff on the page"
you could add an option to hide/show columns!
AdminRob Renaud (Programmer/Parsing Monkey/Fake Statistician, Councilroom.com: Dominion Statistics) commented
My problem with the popular buys page is that there is already too much stuff on the page. I think under/overbuying are good first order things to consider if you do poorly with a card. And you can always eye ball the total effect by doing a weighted sum of the with and without. Further you can sort by win rate given avail, which will be super correlated the measure you want already.