While I appreciate the focus on Aha to promote positive contributions, only allowing 'votes' gives us half the picture. If we have an idea with 200 votes that might seem like a popular idea, but if 300 people don't like it (which at the moment is unknown), then that changes the complexity.
At the moment our users get around this by commenting 'Downvote' in the comments, which is fine, but an actual option to enable downvotes on ideas would mean this could tie into automation, scoring etc.
This is intriguing, but likely requires some nuance. Presumably it's not helpful for downvoting to simply reflect that some number of user wouldn't prioritize a given idea. However, it could indeed be handy to a product owner for users to be able to indicate something like "wait--if you implement that idea it's actually a hindrance to me" (as opposed to simply that they dislike an idea that wouldn't ultimately affect them).