There are many, many ways to contrast Paradise Lost with the other 3 epics of antiquity. The most striking difference is era; Paradise Lost is a 17th century epic poem, written in blank verse, which is a more modern method of phrasing meter. Of the other 3, The Iliad and Aeneid come from the same dactyllic family, while Gilgamesh is more antiquated in its verse.
Other major differences include:
1. Subject matter. Milton uses Bilbical texts and Christian backstories as his subject while the others use mythology and classical notions of heroes and war to build their epics.
2. Format. Milton not only uses blank verse - a precurser to the more modern free verse - but ultilizes dual narratives: the revenge of Satan and the fall from grace by Adam and Eve. Adam and Eve's storyline in fact is quite modern in that it is not played out on the battlefield, but is a domestic, inward family epic, played out with mundane but symbolic actions that lead to the fall.
3. Multiple interpretations. Unlike the ancient epics, Paradise Lost has multiple critical interpretations, especially when it comes to the character of Satan. Some scholars agree see it as a Christian morality tale, while others see Satan in a more sympathetic light. Yes, he is the hubristic antagonist, but his motives are real, his anger is genuinely human, and his motivations are ambitious and modern, pre-supposing capitalism and class mobility.
4. The journey. In classical epics, the journey (to Troy, to Rome, to Uruk) is the prime motivation for the heroes to literally demonstrate their heroism. Paradise Lost is more of an inward journey - in fact, the main journey taken, which is Satan's fall from Heaven, has occured before the action takes place. So instead, we have an epic of plans, of motivations, of ambitions; the climactic actions are mundane in comparison to the bloodshed of Achilles, for example.
5. Side stories and characters. Paradise Lost, while employing a dual narrative, remains focused on these 2 interweaving stories. Classical epics wander, giving great attention to ancillary characters and myths. The rage of Diomedes in the Iliad comes to mind as an example of a tangent that is meant to illustrate the battle and bring glory to a side character, but gives no insight into the central story of the revenge of Achilles. This was done to depict multiple heroes on a broad tapestry, which defined the Homeric or classical epic. Many things are happening at once.
6. Heroism. Finally, classical epics depict heroism as an act that includes vanquishing the enemy through guile, wisdom and maximum bloodshed. Paradise Lost depicts heroism in quiet servitude and willing acceptance.
There are several ways they are similar, and this is more to the point of what defines an epic poem. Epic poems must:
1. Adhere to a metrical verse whether it is dactyllic, iambic, blank, etc.
2. Open in medias res, or in a state of rest where the narrator can reflect on events.
3. Open by stating the cause of the poem. In Iliad, it is the anger of Achilles; in Paradise Lost, it is the ways of God to men, etc.
4. Include many characters who play pivotal roles in the plot at hand. This includes long lists, genealogies and catelogues.
5. A mythological or supernatural element that interacts with men.