Well the problem seems to me that the productivity of a discussion/argument is also dependent on the people you're arguing with. Unfortunately there's very little one can do mitigate someone else's reaction to what you say, especially on a virtual medium. With tumblr, it's really about the ask/fanmail (fanmail, despite the silly name, allows for longer posts) option and a request to have that person respond privately, but again, there's no guarantee that they'll do so. Reblogging with response sort of defaultly gets absolutely everyone involved and there's no stopping other folks chiming in, which can help but more often stokes the fire. Sigh, and now this has become a bit hopeless sounding. Yeah, tumblr's hard to have one on one complicated conversations.