Mill writes carefully and plainly in "On Liberty," and he is seemingly at pains to make his argument accessible to the general reader. He writes persuasively and succinctly, without the ornate, often alienating prose that decorates a lot of the philosophy of this time period (the mid-19th century).
Occasionally, however, he'll slip into more figurative speech (to great effect, given how sparing it is in the essay), as in this description of the growth of the ideals of Socrates and Christianity:
Socrates was put to death, but the Socratic philosophy rose like the sun in heaven, and spread its illumination over the whole intellectual firmament. Christians were cast to the lions, but the Christian church grew up a stately and spreading tree, overtopping the older and less vigorous growths, and stifling them by its shade. Our merely social intolerance kills no one, roots out no opinions, but induces men to disguise them, or to abstain from any active effort for their diffusion.
The imagery of this passage might be grand, but it remains as crystal clear to the reader as the path of Mill's logic in his argumentation. In fact, the effect of such grandiosity is that much greater because it is so sparing in "On Liberty." Passages like this land like exclamation marks after paragraph upon paragraph of Mill's measured rationality.
