UPDATE: see below for the inexactness of words
I know Kos wants to get libertarians to vote for Democrats. Politics makes strange bedfellows (the Democratic Party was, after all, for decades, the party of Northern blacks and Southern racists) but I just plain don't think it's tenable, and if it is I don't want to be here.
If Libertarians start voting for our guys, I -- and hopefully the rest of us -- will view them much the way my nominally rational Republican friends view racists and fundies: pay them lip service and keep them happy with code words so they keep voting for us. If we must, we can look for little bits around the edges that we can implement that wouldn't harm our core values too much; but if their policies ever get enacted in large scale it'd be an unmitigated disaster, and we need to rely on them continuing to be fools if we mean to count on their votes.
What do they care about most? Ask them. Every libertarian I know tells me his top priorities are "cut taxes and prevent gun control". There's already a political party for these people, and it ain't us.
Sorry, but big-L Libertarians are fundamentally antidemocratic, and their core values are incompatible with representative government just as much as Christian fundamentalists' are. If they want to vote for Democrats it's fine with me; a vote is a vote. But they're nutbars, and if they start insisting on a seat at the table and a real voice in policy they could ultimately cause as much damage to us as the theocrats are now finally causing the Republicans. Ultimately, what I hope is that by association with rational people, Libertarians will moderate their positions. I'm not terribly hopeful.
It's not too far off to say "libertarians are Republicans who want to smoke pot and watch porn." While they're anti-authoritarian, and some of them have especial problems with the Bush strain of Republicanism, I haven't noticed that they have much in common with liberals. They may distrust corporations -- but are unwilling to regulate them. They may dislike racism -- but they're unwilling to do anything about it. They may be secular -- but they prefer a world where schools are run by churches.
Read their platform. These guys want to remove worker safety laws, the minimum wage, and the 40-hour work week. They want to abolish the EPA and FDA. (E.coli, anyone? Anyone?) It's the libertarian wing of the Republican Party that wants Social Security torn down. A libertarian's idea of the perfect government reaction to a disaster is New Orleans. (Newt Gingrich is a libertarian. Barry Goldwater was a libertarian. Grover Norquist is a libertarian. No libertarian would disagree with Norquist's intent of drowning the government in the bathtub.)
About the only thing I can say in their favor is they'd probably like John Edwards. Libertarians -- again, read their platform -- believe the way to have personal justice is almost exclusively through lawsuit.
I know the real libertarians don't like the Patriot Act or the war. That's damning with the faintest possible praise -- no patriotic American likes the Patriot Act or the war.
This is a deal with the devil. The Democratic Party that compromises with Libertarians -- beyond the fundamentals of free speech, opportunity, and personal liberty that all Americans, of all political stripes, are supposed to revere -- is a party I won't recognize.
Politics is kind of like an automatic transmission. D goes forward. R goes backward. L causes the engine to explode if you try to go faster than an individual can manage on his own feet without assistance.
UPDATE: Several commenters suggest I'm unfair to tar all "libertarians" with the same brush. Overgeneralization is almost inevitable in a short format like this. Words are inexact, and many people call themselves "libertarian" who are nothing like what I describe. It's kind of like "feminist" that way: the word doesn't necessarily say anything by itself.
Any Libertarian who is already voting for Democrats -- i.e. a "left-libertarian" -- doesn't need to be reached out to. They're already here. The Libertarians I'm talking about are the ones who aren't already voting for Democrats -- the ones who read Reason magazine and that Kos is reaching out towards.
Obviously I could have been clearer.