The interesting question to me is why is it that "pissing off liberals" is delightfully transgressive and oh-so-fun, whereas "pissing off conservatives" is boring and earnest? Based on their writings in Freakonomics 1 and their blog, Levitt and Dubner strike me as open-minded political pragmatists, so it's not that I think they have a big political agenda.
It's possible to write things that piss off conservatives while still retaining an edgy, transgressive feeling--take a look at Nate Silver (or, to take a less analytical example, Michael Moore)--but I think it's a little harder to do. Flouting liberal conventional wisdom is funner somehow. As I said, I think there's something more general going on here but I don't feel I have a full picture of this phenomenon.
Good question! Here's some speculation...
Let's say, as Gelman suggests may be the case for Levitt & Dubner, that you're a reasonably smart person with no particularly strong partisan or ideological axe to grind. Let's further say that you're an academic, a quasi-academic, or live in a major intellectual hub. Is it more fun to tweak liberals or conservatives?
First, right now, conservatives are a mess; picking on them is shooting fish in a barrel. If you want to show that you're clever, or want to just feel clever, then there's not much point to show, for example, intellectual inconsistency in the ravings of Glenn Beck. Of course, there are plenty of intellectually honest and rigorous conservatives out there, but they're so lost behind the dominant Palin/Bachmann strain of the movement that it seems unnecessary for anyone to take them down, and somewhat churlish besides.
Second, given that you live in those areas, you are presumably surrounded by liberals (and true left wingers). Clever people generally love being contrarians, and if most of what you see around you is liberal, than tweaking liberals is the obvious contrarian position.
Third...hmmm, how can I put this. Let's suppose that in general there's equal intellectual rigor on the left and on the right, overall. Let's also suppose that on both sides, there's a wide range of rigor, with smart people who really care about politics and public affairs generally acquitting themselves very well in discussions about such things, but people who are either not as bright or not really all that interested spending most of their conversations about politics and public affairs parroting back the latest that Rush or Olbermann said. Well, if you're on a college campus, or otherwise among the people that Gelman's talking about, you're going to get more than your fair share of the Olbermann groupies, but very few of the Rush ditthoheads. In other words, its not just that you're around more liberals than conservatives (see above), but that as a result you're around more stupidity from the left than from on the right. The result: liberals annoy you more than conservatives annoy you, and so it's more fun to tweak them.
Any other suggestions?