Karl Marx considered 'alienated labor', as he called it, to be one of the major problems with capitalism. People often interpret the phrase to mean that the workers are unhappy, but it actually means something more.
The idea is that the proletariat workers are completely under the control of their employers. According to free market theory, you have an absolute right to dispose and control your property as you see fit. Therefore, the owners have complete control over the work place. They decide what/how/when things are going to be done with their property. The workers, if they wish to be payed, must do as they are told. The workers are free as soon as they leave work, but to earn a wage, and therefore survive, they must sell themselves into slavery everyday.
Not all workers are alienated. Professionals such as doctors, lawyers, and fortune tellers usually have autonomy. They still must work for a living, but they don't have to do it on the terms of an evil, controlling capitalist.
Marx called a person who had to work in an environment which was owned completely by another a Wage Slave, alienated from his job, and believed that this was immoral.