Someone gave an anonymous "thumbs down" to one of my projects, without even bothering to say what it was they didn't like! I worked hard for many weeks on this project, and I offer it for free, with free support. I am really upset and discouraged by this thumbs down. I feel like I have been kicked in the teeth. Why do I bother to offer my software and my time for free? What kind of stupid crazy idiot am I for doing this? It really made me think. I should make better use of my time. I have already dropped my plans to publish one new project and upgrade another.
Compare this with giving bad feedback on eBay. First, eBay encourages users to have exhaustive dialog to resolve a dispute before leaving bad feedback as an absolute last resort. The system warns you that leaving bad feedback is extremely serious and should not be undertaken lightly. And of course, you cannot do so anonymously. And of course eBay is about products that are being bought and sold, so in some cases bad feedback can be justified.
Here on SourceForge, developers work hard and give their time and expertise for free. I don't think that "thumbs down" is even remotely appropriate in this environment. Thumbs up is fine, but thumbs down is wrong here and should be removed. If users don't like something in a project, they should engage in constructive dialog with the author, or better still, offer to help. Just giving a thumbs down is rude and ignorant and we should not encourage this kind of behaviour here.