The biggest problem with liberalism is that much of it is based on incorrect assumptions about human nature, the government, and how the world works. That's why liberal policies that seem so well-intentioned often have such disastrous results over the long-term. Until liberals start to understand these truths, their ideas will continue to be damaging to their country and the world.
Human beings are born selfish and badly behaved: You, me, your father, Mother Theresa, the Pope -- we were all savage, self-centered little animals when we were born and it was only through the socialization process that we became fit for other humans to live with. That's why people do need religion, rules, and social pressure to be convinced to behave. It's also why projects like communism, which assume that people can be brainwashed to completely disregard their own nature and love their fellow citizens every bit as much as they love themselves, are always doomed to fail.
Change is often a bad thing: It goes without saying that sometimes laws, traditions, and moral practices should be changed, but it shouldn't need to be said that change for its own sake is not a plus either.
Unfortunately, there are far too many people who treat massive changes, changes that have the potential to permanently impact our society in a negative way, very cavalierly. Sure, we'll change the definition of marriage, legalize drugs, weaken the Christian church, allow countless numbers of illegal aliens from a neighboring country to become citizens, etc., etc., etc. -- what could go wrong? A lot, actually.
In fact, if you look back through our history, for every positive change on a large scale, getting rid of slavery for example, you can find multiple significant changes for the worse: like killing children by the millions via abortion and massive increases in illegitimate births because of welfare. That doesn't mean we should avoid change. Let me repeat that: that doesn't mean we should avoid change. However, we should take great care before making massive changes to the way our society works.
People are different: This would seem to be a rather obvious point, but it's one liberals seem to have a great deal of difficulty digesting. In their mind, women and men should be treated as if they have exactly the same predilections and abilities. Different races should perform identically well, in precisely equal percentages, in every activity, and any difference between people must be explained by some sort of unfair societal constraint placed upon the less successful.