The sheer bigness of the body features that it created. (Big muscles, thick skin, stocky frame... urgh... so ugly...)
Oh wait, actually, never mind, hair loss. Definitely hair loss. Nothing in my life has given me more nightmares and more of a feeling of desperation and hopelessness than when I started losing my hair. NEVER again. That is the #1 thing that reminds me that I could never go back.