It seems like a good question. Until one considers the source of the complaint--and that rather helps answer the question.

Maybe the "liberal protests" are where the American Prospect's cover story of the "Baucus 13" is.Or the where the American Prospect's lengthy piece on Linda Allision's exchange with Obama is.

Since clicking on the above links yields absolutely zero results in the American Prospect coverage, Husseini urges you to "read up on the 'Baucus 13' and Linda Allison's questioning of Obama" while asking, "If 'liberal' mags like the American Prospect were serious about reform, wouldn't they have relentlessly plugged the 'Baucus 13' and Linda Allison?"

But American Prospect's Tapped blogger Adam Serwer (8/5/09) has a question regarding Milbank's aside that "it's a brutal world out there in the blogosphere.... I'm often surprised by the ferocity out there, but I probably shouldn't be":

What's the sound of a million hands facepalming? No one who goes around using obscenities to describe other reporters and administration officials should be complaining about the "ferocity" of blogs--if Milbank is bothered by it, he might start by admitting his own complicity in creating that kind of discourse.

Serwer's reiteration that "Milbank's unique place in the journalism world entails him making fun of people for a living" yields a simple maxim: "If you can't take it, don't dish it."