Tildes may be an interesting testing ground as a forum without downvote. It has Labels instead for marking less constructive comments, like Joke, Noise, Malice, etc.
Tildes does not rank based on voting, and instead displays votes as a separate signal. IMO it's a great piece of UX. It gives the author the pleasure of imaginary internet points but doesn't allow highly online mobs to sway the ordering of discussion.