The Significance of "Heh"
The difference between the left and right blogospheres is that the left is interested in exploring new ideas and learning what the MSM isn't saying, while the right is interested in a masturbatory circle jerk. That's why Red State eliminated comments that increasied the diversity of ideas on their page. Kos on the other hand eliminated commentaries that were all basically repeating the exact same thing and reducing the diversity of commentary on his boards. The left wants truth, so they are willing to let their ideas be attacked. The right wants isolation, so they insulate themselves.
The reason Instahack links so much is that it allows him to retain his prestige and audience coalition by distancing himself and his brand from the content on his page. When the libertarian Schwarzenegger Republican disagrees with the blockquote Instahack pulled from the bible thumping Dobsonite, he can just dismiss it by thinking "Insty doesn't think that way, he's just pointing it out," That's what the "Heh," is for. He's laughing with it if you agree, at it if you disagree. In this way, he can get away with distributing blatant falsehoods without losing credibility. He's not reporting the falsehood, he's reporting the fact that others are saying the falsehood.
But because Atrios is trying to get important new sorties out rather then repeat the party line, he doesn't really care what a bunch of bloggers siting around in their pajamas think (unless they hit upon a new story). It's just a lot harder to write a post that makes someone go hmmm rather then heh, indeed.