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.