These two beat me to it. Morrison should never, ever, touch Jason Todd again. From the randomly being a red head (doesn't work with post-crisis Jason) to being mustache twirling villain, rather than a vigilante that had a serious hatred for rapists and those that targeted kids (not mutually exclusive, but still), but didn't kill willy-nilly. Morrison's character assassination of Jason still has an effect on the character to this day.
Morrison has done some really good runs on other characters, but was awful when he had Jason.