Ok, Bret inspired me to post this.
No good political philosophy can answer every political question without sliding into dangerous idealism. But we can certainly hope that a philosophy will give us constructive ways to think about every question. As an example liberalism argues that government should give people more and better options if a mechanism can be found to reliably do so. On it's own, this doesn't settle the question over whether we should - for example - support universal health care. But it at least gives us a way to organize our subjective judgments to make a political decision.
In contrast to liberalism, libertarianism seems a much more far-reaching. It says that "I should be allowed to do anything I want, as long as it doesn't harm others". This is a simplification of course. But instead of leaving difficult questions open - like liberalism or conservatism for that matter - it seems to give us exactly the wrong answer on a host of issues.
Is the government allowed to regulate portions of the economy merely because doing so will lead to prosperity? Libertarianism says no. Not unless you can show it involves someone harming someone else. So goodbye universal health care or even preventing insurers from discriminating against those with hereditary conditions. And the federal reserve system is out as well.
But that's not all, because there's nothing in the libertarian formulation that seems to account for the level at which government programs are enacted. So I don't see why street-lights or Bart or Cal trans, or even state parks or zoning laws are allowable under this philosophy. And indeed, there are many libertarians that think just that.
Ok. Now that I've made libertarians angry by over-simplifying their views let me step back and recognize that this probably isn't what most libertarians think. On the occasions that I've made this argument to libertarians their response was basically to whip off the mask and to reveal a totally different, non-libertarian, philosophy underneath. For some it's radical federalism: the belief that somehow local government is better than federal government (which is wrong for a whole host of reasons). For others it's a kind of soft liberalism that dares-not-speak-its-name: people should be left alone unless we can prove there's a really good reason to do so. For others it's good old conservatism: they just want to cut the welfare state to force people to turn to churches and other similar organizations for support.
I don't think you get this with other philosophies. Liberalism doesn't always give you an answer, but at least you don't have to jettison it to get pragmatic solutions to everyday problems. With a lot of libertarians on the other hand, it really does seem like it's just an attitude masquerading as a philosophy.