@AnonymousLlama I like forum software that offers a range of reactions (ala vBulletin / XenForo). The default reaction could be a simple "Like" with an additional click or two required to change the reaction to Informative, Agree, Funny, Sad etc. Two of these reactions (probably positioned last to add that extra little bit of friction) might be Disagree and Off-Topic.
But if this is too convoluted, maybe just cap the number of downvotes a post/thread can get. So if a post gets 5 (or whatever the cap number is) downvotes, additional downvotes do not change the downvote counter. This would discourage "piling on" but there'd still be an indicator that a post might be poor quality or contentious. On the other hand, upvotes should probably be unlimited (or have a higher cap number) to encourage positive feedback.