Milton's style is first of all epic; he is consciously writing an epic poem, modeling it after the great epics of the classical past.
His language used is Latinate; he considered writing the poem in Latin, and adapts many terms from Latin. This makes his style seem more formal, and makes many terms more complex than if drawing on more familiar English words.
It is written in blank verse, and iambic pentameter.
He uses many allusions to classical and exotic topics, adding weight and grandeur to the style.