The things that make a game epic for me are: good sound effects, fitting music, and camera movement. Explosions and destruction of course add to the epic of any game, as well.

As an example: if you have Doom 3, I would suggest you download the Shambler's Castle mod and give it a run. The background music, lightning, cutscenes, and sound effects are absolutely superb. The music especially really makes me feel like a super hero or something. To me, that mod is extremely epic.