McMurphy's unusual in that he doesn't conform. Everyone else in the institution subscribes to its petty rules and regulations without a moment's hesitation, but not Randle. He follows the beat of a different drum; he answers to himself and no one else.
That includes Nurse Ratched. All the other inmates are somewhat scared and intimidated by her; but once again, McMurphy breaks the mold. He's prepared to challenge and undermine her authority at every available opportunity.
Nurse Ratched's dealt with some pretty difficult patients over the years, but none quite as challenging as McMurphy. She realizes early on that he's a serious threat to her authority and to the smooth running of the institution. That's what makes him such a danger. And that's what makes him so unusual.
What isn't unusual about McMurphy? The word "unusual" takes on a much different meaning in a mental institution. I spent the first half of the book just trying to decide whether he was truly mentally ill or not. Or whether many of those inside were. Clearly, some of his compatriots needed to be institutionalized, but McMurphy was a match for Nurse Ratched, and the others enjoyed watching them go back and forth, with McMurphy winning a good deal of the time. If he did have mental issues, they didn't affect his intelligence or awareness of himself and those around him.
His voice and laughter were commanding, they had a presence that gained attention, both positive and negative from those around him and the hospital staff.