Libertarianism is bad because it is a grotesque combination of Social Darwinism and Capitalism's worst tendencies. It assumes that all people start life with the same potential at the same socioeconomic level and then decries those who can't measure up.
Most libertarians have it pretty good all in all, and more importantly came from places where the thinking is "If you just work hard enough, you'll catch the right breaks and you'll be successful." This isn't a bad sentiment in itself (hard work does pay off in one way or another), the issue is they can't fathom that their parents station in life determined their station in life - "Well just go to college and then get a job!" doesn't really work in the South Bronx or Brixton or whereverthefuck ghetto you can think of. There are success stories, but more often than not the story is "look at this bright young black child overcoming her awful surroundings".
It's great to be able to take advantage of opportunities. Nobody has an issue with people using their talents to better their life. Libertarians assume that the same opportunities are always available to everyone, and it's fucking insane. Also they hate paying taxes and are content with making everybody else pay more so they have to pay less. It's selfish and it's a slap in the face to democratic society.