I disagree with the last one. As someone who's been involved with quite a few forums and social sites, the down button tends to get abused a lot less than you'd think. Heck, the most popular plugin for XenForo ratings goes further and lets people mark replies as dumb, old or 'drama queen'. Those don't seem to get abused too much either, surprisingly enough.

But I guess it's a size and topic thing. If your site has a mature userbase, a limited topic or is mostly a pretty tight knit community, adding a downvote button tends to be a decent enough thing to do. Same if you have to acquire the privilege to use it.