A recent interview with VALVe's experimental psychologist details VALVe's play testing methods (http://uk.xbox360.ign.com/articles/966/966976p1.html). They're more complicated than you'd think; muscle tension, blinking rates and facial expressions all send messages that can improve the game. This rigourous play testing is somewhat harder for modders but still it shows how importans face to face feedback is; online playtesters just don't cut it.