I don't care much for the droopy look, whether its in eyes (and I have HUGE eye issues myself, eyeball in general makes me sick) or droopy lips, which kind of knocks out most mastiffs. I also dislike blue eyes, though that isn't the make or break for me.
Um, I don't like smushed faces in dogs, but love it in cats.
Dislike the look and feel of poodle/doodle coats.
Of course, I never cared for merle coloring, either, and look who I ended up with.