I gotta go with ZEB on this one. I love the show, but I do get sick of all the liberal agenda pontificating during some of the more topical episodes, especially the preaching that Brian the dog carries on with. Some of the commentary is overtly political and clearly takes a stance on certain issues. When I find myself in disagreement with these well-defined points of view, it turns me off some and it isn't because I turn everything into a politically-framed debate. The show forces these sorts of reactions on to some of their viewers.
Some of their blatantly pro-left wing agenda stances are entertaining because I agree with some of the assertions being made, but they're just telling me what I want to hear. Other times, I totally disagree because they are not telling me what I want to hear. I see this sort of thing from the conservative media/entertainment industry as well though, so it's not a symptom of liberalism by itself.