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.