The liberal-libertarian alliance seems like a natural one given the way that conservatives are stereotyped as being in favor of regulating all manner of personal freedoms and rights. On balance, that's not a fair comparison, especially given the way that liberal views on economics and some social issues tend to have a greater tendency to increase the chance that rights will be violated.
Conservatives and liberals both resort to the government to solve social problems, but liberals are more likely to resort to government because they see a far greater number of social issues to fight. Liberalism has brought a great deal of intrusiveness into employer-employee relationships, given us regulation for every aspect of production, and imposed taxes on virtually every conceivable source of revenue. That is to say nothing of redistribution of wealth, entitlement programs and government subsidies for everything from the arts, to private "charities" which are doing work that some deem of public interest.
Economic freedom is not as simple as the freedom to keep one's income, and to buy goods and services. Rather, it encompasses even the very right to create itself. Every time that someone takes a product and modifies it, changes their house, builds something new, starts a business or carries out a private transaction, they are using economic rights. The liberal tendency to want a government override or "safety valve" controlling "excesses" here should be deeply troubling to libertarians because the "personal rights" that liberals generally want no government regulation on are a smaller subset of our rights than economic freedoms.
From a libertarian perspective, the only rights which liberals typically respect are keeping the government off of your body, freedom of speech and due process rights. However, many middle and upper class liberals fall down here as they too support limits on freedom of speech which are of dubious value and will support the War on Drugs, but with less gusto than many conservatives.
If that still sounds tolerable, then there is one unforgivable sin that liberalism has committed, and that is its systematic violation of the rights of children and parents through the public school system. They are the ones who created the modern, socialized, compulsory system of education. They are the ones who stripped students of their right of self-defense, who have destroyed school curricula in the name of tolerance, diversity and equality, and they are the ones who have worked tirelessly to defeat alternatives like school vouchers and homeschooling. The public school system is an embodiment of the fundamental difference between libertarian and liberal values.
Libertarians and other minarchists need to realize that the liberal worldview is fundamentally, irreconcilably statist in a way that conservatism is not. It would take an entire book to do this subject justice, in order to account for all of the nuances and flavors of the political left and right. Jonah Goldberg's Liberal Fascism is probably the best starting point that I could recommend.
The real travesty of public-schooling is that it is a self-perpetuating parasite, so long as it fed on the blood of the taxpayers; I'm going to use this as my default acid-test for conversation incident to political interests from now on.
The sin is not only grievous, but its proponents are often intractable in their ignorance, which solves little in a protracted discussion.