In looking for material related to this topic I ran across an interesting article called "'Warring Chains of Signifiers': Metaphoric Ambivalence and the Politics of Paradise Lost" by Peter C. Herman, which focuses on the ambiguity behind many of Milton's metaphors, especially in describing Satan. I think this ties into the question: Is Satan the "hero" of Paradise Lost? While this article doesn't answer that question directly, I believe it provides some insight as to why this remains such a long-standing and divisive question.
Herman, in his essay contends that PL "includes a fundamental, unresolved incertitude, whether conscious or not, about the Republic, kingship, Cromwell, and the tenability of rebellion" and that Milton "invokes comparisons that disperse certainty and provoke mutually exclusive interpretations" (268-269). Herman gives several examples of metaphors used in describing Satan that seem to have disparate connotations depending on how you look at them. For example, Satan is compared in Book I with a Titan from classical mythology called Briareos, who (depending on the source) was either an evil character that was rightly thrown out of power by Jove or a hero who actually fought on the side of gods (271-272). This throws into confusion what is Satan exactly in this metaphor--is he a "hero" or is he a villain?
Herman closes the article asserting that the ambivalence behind interpreting the character of Satan is Milton's way of trying to work through his experiences. He writes of Milton:
Having witnessed the Republic betray its principles and collapse, then having to endure both the Restoration and the ensuing danger to himself, Milton created a text in which he inscribes the terrible oscillation between assertions and subversions of principle that results when everything has unraveled and one is left trying to make sense of the ruins. (285)
This is just one article, so I'm definitely interested in pursuing this line of thought.
Herman, Peter C. "'Warring Chains of Signifiers': Metaphoric Ambivalence and the Politics of Paradise Lost." Texas Studies in Literature and Language 40.3 (Fall 1998): 268-292. Electronic Version.
(A copy can be found on JSTOR at http://www.jstor.org/stable/40755264.)