What conformation fault annoys you the most and why?
As a distance rider, I've been brought up to hate any type of leg fault, because they are detrimental to the soundness of the horse over the long run. I have a special dislike for weak pasterns and back at the knee/over the knee. Weak legs an endurance horse does not make.
However, I don't mind a mild cow hock (especially on a gaited horse). It doesn't seem to mess things up too much.