My two cents as a non-piper, just listening:

I have a friend who is a top notch piper with credentials up the wazoo, many prizes and awards for her talents. We will be somewhere and she will say,"That is a really bad piper." My response is usually "Really?" At this point, my pipe music ear can't tell the difference. I am learning tho and am beginning to pick up and some of the really major errors. Still and all, I have a hard time knowing what is good playing and what is bad overall. I guess it is something you need to be around for some time to begin to know what is good or bad. On the other hand, we were at a German Fest and i commented on the band that was playing saying it was a really bad ompah-pah band - her response was "Really?" I guess it's all in ones background.