(I aslo wrote this awhile ago and am uploading it now) I have just finished the book. I think the book asks a few main questions which I would sum up as:
1. What constitutes as insane, and how is that decided?
2. How does society influence/ control us, and more importantly should we let it?
3. What role do impulses and desires play in human life, and why is are they important?
All of these questions sort of tie back into the main idea of following and accepting what is set out for you vs. acknowledging your inner self and figuring out how you feel as a bases for what decisions you make. The emphasis is on ones ability to not blindly follow. (Though personally I also see the patients follow McMurphy which is still following)

In terms of insanity, I think the book brings into question the idea of how and why it is decided that someone is insane. McMurphy is not what one might classify as classically crazy, but he is admitted, and given intense therapies (electroshock, and a lobotomy). In the story asking questions marks one as insane, and someone who needs to be "taken care of" like Maxwell Taber who was given electroshock treatments to make him docile simply because he asked what kind of meds he was taking. Nurse Ratched is in power, and if she see a threat in you, you will be immediately classify you as insane. It is easier to remain in power if you denote anyone who questions you as crazy. The book insinuates that those in charge of our societies, have discovered this as a way to keep people quiet and obedient.

In thinking about the book, most often society is said to be what influences us, what controls us and makes the decisions, But who/ what is in control of this "society",  how do they influence us and should we let them? The book uses Nurse Ratched and the ward, to represent a bigger picture of society as a whole. The obvious controlling factor in the book is Nurse Ratched, she runs the ward like a dictatorship, and controls the hospital or the "society". As I read more, I started to notice more and more mechanical imagery, used as negative symbols. Machines belittle the individual and help make everyone become part of the whole (the whole being Nurse Ratched's ideal ward, or Kesey's view of the modern society) Nurse Ratched's aids are described as mechanical, and they keep the patients at bay. The fog Bromden see's obscures his judgement and ability to see the world, at times he considers it safe as it shields him from his reality. He also believes this is not naturally occurring as fog usually is, but is a result of a fog machine controlled by the staff and Nurse Ratched. This is blatant use of machinery as a controlling force to stomp out individuality, by clouding one's view of the world. Kesey uses machinery to show the repression we may not even realize we face in modern society. Society has eroded our self awareness and natural impulses. Kesey suggests that we should refuse to allow society into our consciousness, and instead embrace our natural impulses and make decisions for ourselves.

As humans we all have urges and desires. With this book Kesey implies that we should not ignore those, they are part of a human experience. If we ignore our desires, who are we? Certainly not happy. Kesey's work poses the question to the reader "Who are you really? What do you desire? Are you content to let someone tell you that this it is innately wrong to have desires?" It is important to recognize what you desire and sometimes act upon impulses in order to experience a authentic human life.