Google Doc-Style Comments
A feature which I have considered for text-based roleplay to avoid quote-pong is a comment feature. In Google Documents, it is possible for viewers to make suggestions to a piece of text by selecting that text and then adding commentary on that particular line.
I think that this could be useful on a roleplaying website as well (though I realize that in the current architecture, this is not possible). The comments would be of tweet-length, so 125 characters or so long. Something like "He will remember this" or "His expression darkens upon her words" or "As she did this, he jumped behind cover".
Short, simple responses to complicated actions without devolving to quote-pong (players keep quoting each other sentence for sentence in a constant back and forth that's not getting anywhere) and without breaking chronology ("would have been having already done under provision X, Y, Z, if X doesn't do this in future").
You could then also remind a poster of any comments they made on other people's post since they last posted, as inspiration for their current post, building off of the short, but sweet comments into elaborate, eloquent descriptions of complex emotions and actions.
I think that this could go a long way in some of the paralysis that I see happening in some games, with players waiting for others to go first so they have more material to respond to. With this method, you can respond right away and everyone will immediately see that "he will step in to keep her from doing this thing" and incorporate it in their own replies, such as "as they fight for control over the device, my character will blahblahblah".
I think it could be a neat way of maintaining the flow of a game.