I think it's pretty clear that judging how Dominion cards play without actually playing them can be inaccurate. However, I don't have any good methods for playtesting cards.
I can't make very good proxies because I don't have that much good cardstock. Can't use isotropic, because it doesn't support custom cards. And using general CCG software works, but is pretty annoying to set up and play with because you're forced to do all the rule enforcing yourself.
The point being, is there a nice way to playtest cards I haven't seen yet?