1. Top-level comments should introduce arguments; responses should be responses to those arguments. 
2. Upvote and downvote based on whether or not you find an argument convincing in the context in which it was raised.  This means if it's a good argument against the argument it is responding to, not whether or not there's a good/obvious counterargument to it; if you have a good counterargument, raise it.  If it's a convincing argument, and the counterargument is also convincing, upvote both.  If both arguments are unconvincing, downvote both. 
3. A single argument per comment would be ideal; as MixedNuts points out here, it's otherwise hard to distinguish between one good and one bad argument, which makes the upvoting/downvoting difficult to evaluate.
4. In general try to avoid color politics; try to discuss political issues, rather than political parties, wherever possible.

As Multiheaded added, "Personal is Political" stuff like gender relations, etc also may belong here.