Here's a user story:
As a member of a planning team viewing a list of cards in order to vote on them,
I want to be able to indicate a negative vote
so that it's easier to tell between cards that are just "not currently high priority" from cards that members actively want to close.
Other than adding comments, there doesn't seem to be a way to apply and tally negative votes on cards. There is no way to down vote currently, right?
UPDATED:
Possible workaround inspired on one of the answers to this: Is it possible to add story points and velocity tracking to Trello?
Each person who wants to downvote can increment a number in the title, possibly prefixed, like "NO(2). This is the title" (meaning this has 2 downvotes). I still don't think this is optimal, but at least it is on the "front" of the card.
Best Answer
Current there is no downvote functionality in Trello.
As a workaround, perhaps you could use a label like "Candidate for Deletion"? Then (as you say) folks could use comments to indicate that the support closing the card.