Liberal media outfits do loves them their unhinged-leftist hate-mail. Because they use it again and again as proof positive that they must be reporting straight down the middle to incur so much wrath from the left.
Although Allah points out the speciousness (and convenience) of that claim, let's also note that most of the liberal media's criticism on the right comes from mainstream Republicans representing the great mass of right-leaning thought, whereas those who think the WaPo is a part of the Vast Right Wing Noise Machine are unabashed, unhinged lefties, "undecided" voters only the sense they're undecided between Ralph Nader and Hugo Chavez.
Which means that the criticism of the paper comes from the hard left and center-right, which means, in turn, the paper is somewhere in between. And what is in between center-right and hard left? Ah yes: establishment liberal. Which is what everyone who's not batshit crazy understands the WaPo is, its support of the Iraq War notwithstanding, and its quite-praiseworthy attempts towards true political balance notwithstanding. Yes, the WaPo is less blatantly agenda-driven than the NYT, but it is still, on the whole, establishment liberal, and its reporting reflects that deep institutional bias.
There's more about the NYT in all of this so click the first link.