I am fine with political discussion on Facebook. Some people love discussing it, and while others don't, they're free to skip over it. I think people really should try to keep civil, but that goes without saying. However, I have absolutely no patience for people who post something inflammatory, then get angry when people disagree.
If you post something political/controversial, be prepared to discuss it! That doesn't mean you need to accept abusive replies, when you weren't abusive, but it does mean you don't get to post something super political, then act shocked and upset when someone politely disagrees.
And since some people hate political discussions, it's rude to go onto someone else's wall or status and make something political.