Plus, as someone has already pointed out, "How do you ensure dis/likes are unique if it's anonymous?"
Does it really matter? Who is going to try and "game" your like count by deleting cookies or other such methods?
But what about vandalism and trolldom, right?
I think this also applies to FB and G+... but I don't care, I want this to be better than FB and G+ likes, which I despise haha. And for a personal photo gallery it is probably perfect, and other such things. Small communities of nice people, which do exist. But for a site a la reddit or youtube or political blog aggregators? Hmm. But maybe that's okay, maybe huge sites can't have nice things, and that's fine.
I hate to sound like I'm dissing this. It's really not meant that way, to the contrary: I love how configurable and sexy it is, I upvoted the submission instantly, and then proceeded to ask questions.
And maybe it's really more a social than a technical problem. In an ideal world, we would use these as tools to make our opinion known, appreciate them as such and never ever fuck with them.