Having just downvoted a feature request here on meta, I saw the tooltip and thought exactly the same. I'd go as far as to suggest that the content of the tooltip on meta sites should be contextual, based on the "special" tags such as feature-request that have been applied to the question.
I doubt that it'll ever be considered a high priority piece of work within the UI, but it would be a little bit "tidier" if the tooltip for a feature-request post downvote read something like "please consider explaining why you disagree with this feature request". One of the "last 1% polish" that the UI would need.
