There are many interpretations of "the dungeon" in D&D. OD&D, in particular, lends itself to a certain type of dungeon that is often called a "megadungeon" and that I usually refer to as "the underworld." There is a school of thought on dungeons that says they should have been built with a distinct purpose, should "make sense" as far as the inhabitants and their ecology, and shouldn't necessarily be the centerpiece of the game (after all, the Mines of Moria were just a place to get through). None of that need be true for a megadungeon underworld. There might be a reason the dungeon exists, but there might not; it might simply be. It certainly can, and perhaps should, be the centerpiece of the game. As for ecology, a megadungeon should have a certain amount of verisimilitude and internal consistency, but it is an underworld: a place where the normal laws of reality may not apply, and may be bent, warped, or broken. Not merely an underground site or a lair, not sane, the underworld gnaws on the physical world like some chaotic cancer. It is inimical to men; the dungeon, itself, opposes and obstructs the adventurers brave enough to explore it. For example, consider the OD&D approach to doors and to vision in the underworld, as described in Vol. III of the original rules:
Generally, doors will not open by turning the handle or by a push. Doors must be forced open by strength...Most doors will automatically close, despite the difficulty in opening them. Doors will automatically open for monsters, unless they are held shut against them by characters. Doors can be wedged open by means of spikes, but there is a one-third chance (die 5-6) that the spike will slip and the door will shut...In the underworld some light source or an infravision spell must be used. Torches, lanterns, and magic swords will illuminate the way, but they also allow monsters to "see" the users so that monsters will never be surprised unless coming through a door. Also, torches can be blown out by a strong gust of wind. Monsters are assumed to have permanent infravision as long as they are not serving some character. (The Underworld & Wilderness Adventures, pg 9)
Special Ability functions are generally as indicated in CHAINMAIL where not contradictory to the information stated hereinafter, and it is generally true that any monster or man can see in total darkness as far as the dungeons are concerned except player characters. (Monsters & Treasure, pg 5)
Notice that all characters, including those which can see in normal darkness (e.g. elves, dwarves)*, require a light source in the underworld, while all denizens of the place possess infravision or the ability to see in total darkness. Even more telling, a monster that enters the service of a character loses this special vision. Similarly, characters must force their way through doors and have difficulty keeping them open; however, these same doors automatically open for monsters. This is a clear example of how the normal rules do not apply to the underworld, and how the underworld, itself, works against the characters exploring it.
Of course, none of this demands that every dungeon need be a mythic underworld; there could be natural caves and delved dungeon sites that are not in the "underworld" category, and follow more natural laws. Nevertheless, the central dungeon of the campaign benefits from the strange other-worldliness that characterizes a mythic underworld.
A mythic underworld should not be confused with the concept of the "underdark." The underdark concept is that of an underground wilderness composed of miles of caves, tunnels, delved sites, and even whole underground cities. This is a cool fantasy concept, but is distinct from the concept of a mythic underworld that obeys its own laws and is weird, otherworldly, and apart from the natural order of things. (There is no reason a referee couldn't join the two concepts of underworld and underdark, though.)
Some common characteristics and philosophies for a mythic underworld dungeon (keep these in mind when creating your dungeon):
* this ability is not specified in the little brown books, but is found in Chainmail