What Mill wants for the individual is freedom within society. This means, for Mill, that the dissenting opinion is respected as much as the opinion which accords with the majority. Law was once the codification of the prejudices of the ruling class, whether the Spartans, princes, nobles, etc. With the advent of popular rule, the risk becomes not just the tyranny of one over another, but the tyranny of the majority over the dissenting individual through the codification of popular prejudice as law. Mill believes that this owes to an lack of rationality, an inability to put aside one's own personal preferences and consider what is most useful for society. In his view, the only legitimate reason to limit the freedom of an individual is the case in which they would do another harm or have harm result due to their inaction. This serves to maximize liberty, because it allows for a great amount of individual liberty while also ensuring that the liberty of others is protected against actions of others which might serve to compromise their liberty.
The largest apparent threat to the individual, in this essay, would therefore seem to be the limiting of liberty without legitimate cause, which results in an inability to live one's life as one would see fit; while coercion seem implicit in this, other forms of limitation of the individual life which are more indirect would likely also be considered threats to individual liberty. The limiting agent, society at large, therefore also appears as a dire threat, inasmuch as large groups of like minded individuals are capable of suppression of dissent and, hence, great injustice through the limitation of individual liberty.