The very significant role that the unconscious plays in human behavior was probably Freud's most important "out there" idea, and it is obviously not so out there any more. Why Freud got so interested in sex is an interesting question, and while the jury is indeed still out on that one, its probably fair to say that sex plays a more significant role in behavior than people were inclined to grant at the time Freud was writing (and perhaps still?). The relation between the neurobiological and the Freudian unconscious is one of the most interesting areas for exploration at the neurobiology/psychoanalytic interface. For more on that see

• Making the Unconscious Conscious: A Bi-directional Bridge Between Neuroscience/Cognitive Science and Psychotherapy
• Who's Afraid of Emily Dickinson? or How I Learned to Stop Worrying and Love the Brain
• Looking to the Brain for Psychotherapeutic Insights and to Psychotherapy for Insights into the Brain