It is the convention that in Meta one can downvote to simply say "I disagree with this post", without there being anything wrong in terms of content, research etc. in the post itself.
When downvoting on this Meta site (and I suspect all others as well) users get a popup bubble urging them to leave a comment on how this post could be improved, or something to that effect (try it!). Also the hover messages are the same as the SE sites, which doesn't do anything to help.
That messages are contradictory to the purpose of downvotes on Meta sites, as described in my first paragraph, and potentialy confusing. I propose the message gets changed into something that urges the user to write an answer on why they think the idea presented in the post is wrong, or get rid of it on Meta sites alltogether.