I think one issue with approach by up/down voters is that sometimes the "thoughtful and substantive" thing to say is inherently ideological, and I notice that being explicitly ideological (for example, making it clear that you regard a long term trend in the economy as serving the interests of a particular group of people) is viewed as counterproductive.
This mirrors a dilemma out in the "real world" where people find it hard to bring up substantive political ideas because just doing that is viewed as divisive and antagonistic. Unfortunately, sometimes (maybe even often) this is actually what is required to have "thoughtful and substantive" discussions.