Milton wrote Paradise Lost at a time when tastes in English poetry were in transformation, partly as a reflection of the political changes that had taken place with the Restoration of the monarchy. He was essentially out of touch with the latest trends. The poem is in blank verse, at a time when rhymed heroic couplets were becoming the standard.
In English, unlike in the Romance languages and German, rhyming verse often takes on a sing-song quality which many readers deem inappropriate to the most serious subjects. By choosing blank verse Milton avoided this possible pitfall. Milton's subject is the biblical Fall of Man—for the religious, the most serious topic one can deal with. He self-consciously presents this as an epic, but one that is even more important and thus artistically more grave and powerful than the epic poems of Homer and Virgil. To state in the opening lines that one is "justifying the ways of God to man" is to let the reader know that what follows is intended as a work that will out-do everything previously written.
Milton's sentences are long and complex at a time when, as stated, English poetry was changing and becoming simpler and more direct, as in the works of John Dryden. It is often difficult to follow Milton's complicated mode of expression. He was a scholar of classical languages, and much of his phraseology seems carried over from Latin into English, giving an austere remoteness to his style.
Another factor in the weighty, serious character of the poem is the depiction of Satan. In the first two books of Paradise Lost Satan, though he is the "arch fiend," is given language to speak and is described in a manner more befitting a hero than a villain. This was a factor that endeared Milton to the anti-religious Romantics a century and a half later, such as Blake and Shelley. But even looked at objectively, the perception of Satan as a powerful adversary, like a negative superhero, enhances the power of Milton's poem. The length of the work overall, the startlingly rich and elevated language, the use of blank verse, the complicated sentence structure, and the explicit announcement at the start that this poem will essentially do something never before attempted by a poet, all contribute to the power of Paradise Lost.