On a similar thread a few months ago someone commented that another discussion forum they use requires one of a handful of pre-defined reasons for a vote (both up- and downvotes). It's a simple dropdown so it doesn't add a lot of friction to voting. Different reasons have different impacts on the commenter's reputation score (for example "hate speech" has a greater penalty than "low quality" and stuff like that). I can't recall what the site was called though.