/r/moderatepolitics on reddit is halfway decent. It's definitely liberal, but the way people discuss things is moderate, if that makes sense. It's not complete partisanship, and occasionally beliefs get questioned.

Reddit and Hacker News both solve the problem of "how do you host a site to allow for political discussion?"

To get good debate, you have to curate a social community by effective moderation. You see it on reddit all the time -- some communities are utter trash, and some are much better. It's a social problem, not a technological one.