This is a really good question. I have thought about this before.
I think that a liberal is someone who views the world as being uncertain and that there is not much truth that we know of. They show this in their actions and words.
So someone who wonders why the Church changed it's position on usury, slavery, geocentrism and religious liberty is a liberal? [etc.]
The point of ggreg's comment quoted above and of all those that follow it is excellent. Here, as on the Boloki book-burning thread, irrelevant hypotheticals and semidefined situational descriptions are standing in for a serious examination of fundamentals.
That being said, the essence of liberalism lies in its dependence on criminally misplaced idealism, which people who espouse liberalism, either sincerely or only notionally, take as putative justification to play God (or as we say here in NYC, to play Bloomberg) with society and with those unfortunate enough to be subject to their (by their own lights) absolute authority. In short, it is a philosophy of government founded upon unexamined and largely undefined human
aspirations rather than on insight into and centuries' experience of human
nature. What is more, liberalism's proponents regard those misguided aspirations as trumping everything else: individual rights, appropriate political and moral freedom, and even life itself. Needless to say, the Catholic principles of subsidiarity and morally dictated limits on the exercise of terrestrial authority are not even included in the mix.
Practically speaking, an English writer named Tobias Langdon, in a recent column attacking Tony Blair's betrayal of his own people and treasonous support of Israeli aggression and Jєωιѕн interests generally, expressed the problem very succinctly: "Liberalism is based on lies about human nature and is destroying the West with its lies."
That's an assessment that can't be undermined either by wisecracks or by seriously stated contrary examples.