I just wanted to add my voice for wanting this feature.
It would be really good if users on WP could simply register/sign in and comment directly under a post on WP without having to navigate away to the forum, so that the whole thing feels more like a commenting system. Their comment would appear both beneath the post and on the discourse thread that was created. And they always have the choice of posting either in discourse, wordpress or both - seamlessly. I have no idea how this would be achieved, but that would be a really seemless way of integrating WP comments with Discourse that feels less clunky and would no doubt vastly improve the number of comments than we currently get with the existing plugin.