I was going to post in another thread, but seeing as this would have been veering even more off-topic than the thread already was, and the thread is about threads veering off-topic, I thought I'd better start a new one ;-)
I do like the idea of being able to give a post an "up" vote.
A down vote option only encourages colliseum type behaviour (you know, looking for blood etc.), as people have said already. But I often miss the option to give a very casual +1 to a post - good reccomendation, nice retort, whatever.
I wont lose any sleep about this one, and am not even sure it's a good idea, but I still miss it - often enough too.