Distilling a thought I left in a comment elsewhere, I have finally realized that the essence of liberalism is diversity of thought and action.
That is to say as long as your thoughts and actions meet the politically correct criteria as established by the liberal elite.
For instance, we need an open mind regarding gay marriage -- as long as you support it. We need a dialogue on race, as long as you are willing to shoulder the white man's burden -- which in modern times is collective guilt if you are white.
All hail diversity! Now get in line. The last century is historical proof that the natural result of progressive liberalism ideology is totalitarianism.