What are the qualities typical of an epic poem?

The main issue with trying to describe the nature of the
epic is that the term is applied across a wide variety of poems from many different
cultures. The Greek term ἐπικός was originally used to refer to long narrative poems
performed orally, including the Homeric epics and the works of Hesiod. These two authors
alone illustrate the variability of epic forms, with one focusing on historical
narrative and the other on supplying information (e.g. the gnomology of Hesiod's "Works
and Days"). 

Within an oral-traditional culture, the poems
(or epics) that are performed and transmitted across generations are ones of cultural
importance. These may include ones which versify agricultural or religious calendars,
stories about gods and heroes, or other culturally important themes. Many epics have a
strong narrative component, but do not necessarily follow the rather restrictive
Aristotelian dramatic model described above. The necessity of oral composition means
that many examples of oral-traditional epos use extensive
repetition, formulaic elements of plot, scene, and story, flat characters, and concrete
world views.

The advent of literacy expanded verse genres
in many cultures, and epic based on received traditional models developed many variants.
Long didactic poems remained common, ranging from Lucretius' "De Rerum Natura" to Pope's
"Essay on Man". As well as traditional heroic epics such as the Norse sagas, one also
encounters mock epics such as "Orlando Furioso" or "Don Juan", extended epistles in
verse, and modern long poems such as Walcott's "Omeros". Although some have a
traditional narrative arc, many do not.

Although one can
talk about typical features of selected groups of epics, such as oral-traditional heroic
epics or mock epics, eventually the only universal quality is that they are long poems,
and that conventions vary depending on period and cultural
context.