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.
A liberal is someone who believes in things without any evidence. Basically someone who believes in what they hear in the media, and so forth, without holding any constructive criticism/skepticism.
A liberal is someone who makes decisions based on feeling rather than logic.
A liberal is someone who believes in what is easier to believe then rejects the truth when it crosses their path because it goes against their agenda.
A liberal is someone who will lie in order to advance an agenda. They think that is ok to lie because, in their minds, their agenda is what is right.
Liberals are generally anti religionist because they believe that religion causes war, and we are better off without religion.
A liberal believes in the idea that society needs to progress past the way things used to be. In their minds, this is for the betterment of society. Generally what these "progressive" ideals lead to is bad things. They just don't realize it nor do they want to.
A liberal is, a lot of times, a hypocrite. They accuse you of things that they are guilty of themselves. When they accuse you of something, it is generally them the ones who are doing it.
A liberal is stubborn, and does not live by the truth. They make a lot of false accusations.
*Note: All of these or some of these may apply to liberals. This is not to say that all liberals hold all of these qualities.