More on Andrew Gelman's question (why, he wonders, is tweaking the left more fun than tweaking the right):
Another possibility is that the current crop of liberals are a lot more likely to engage on the merits than are the current crop of conservatives. Write a debunking of climate change denialists, and you're likely to get silence from the conservative blogs, or at best a sort of "there goes another liberal" ad hominem response. Write a debunking of environments, and you'll be met with chapter and verse of exactly why you're wrong, which allows you to response substantively, which is good for one's self-image as a clever person, and really just far more interesting than the "I'm not a partisan, I'm just giving the facts" rejoinder to the conservative response.
But really, I just wrote this follow-up so I could link to my special transcript of Obama's speech to schoolchildren last month.