The liberals treat the hard leftist icons — say Saul Alinsky — reverentially and with warm nostalgia. They welcome former radicals into the ranks of respectable liberalism with open arms, very rarely offering a hint of concern that they once cheered the Black Panthers or some such.
The problem is that the entire cornerstone of the neoconservative movement is a contingent of former Trotskyists who have been embraced by the greater conservative movement (though the embrace seems to be weakening quickly). Not only were figures such as Burnham and Kristol radicals in a past life, they became radicals in a new life - espousing a new kind of 'permanent revolution' on the international scale. Now, I'm all for giving people a chance to renounce their past if their past was indeed an unsavoury one, but it's worth pointing out that the original neoconservatives never renounced their radicalism.
And if you want to try to taint an entire movement by pointing out its worst members, as Goldberg is trying to do to liberalism, you could counter by questioning why mainstream conservatives treat Limbaugh, Coulter and Malkin as serious thinkers who deserve to be listened to.