"No one can be a great thinker who does not ... follow his intellect to whatever conclusions it may lead". Mill's forceful advocacy for freedom and liberty has two central themes: firstly that society has no commission from on high to intervene in any person's life except where society might be harmed. Exactly this last condition becomes a slippery ground for Mill when he later tries to clarify the line between one's own business and that of others. This is not an easy point and Mills does as well as one can expect. The second theme is that since no one possesses all truth (obviously except for religious frauds or totalitarian despots) all diversity of opinion and expression of living must be encouraged because that's how we shall uncover more truth and remain vital. Almost all of the book is a powerful polemic against the crushing of individuality and character in people who hold "unpopular opinion". As Mills goes on to say: "Who can compute how much the world loses in the multitude of intellects who dare not follow any bold, vigorous, independent train of thought." There is more reasoned intelligence and current debate in this book than in virtually any newspaper you care to read today. The arguments against bigots "who consider as an injury to themselves any conduct they have a distate for" is still right on the mark 150 years after being written down.
I think On Liberty may have done for me what I had sought in all those positive thinking books gathering dust on my shelves. OL is a fulsome, nay winsome, celebration of individuality and strength and the joy of being different and seeking happiness in one's own way and to one's own taste. I am not (nor is anyone else) a statistic estimator to be ground down to within zero variance of the mean.