1. A good map. The map is an important part of the dungeon. Thirty feet of passageway to room with orcs, followed by thirty feet of passageway to a room with kobolds, just doesn't cut it. Maps don't need to be Escher-like studies in insanity, but they do need to have enough caracter to make the players feel like they are going somewhere and not simply visiting the same room over and over again, or just following a straight path to the end of the dungeon.
2. A sense of scale. Dungeons in the old school sense were big places. Individual rooms might be small, but the overall impression should be that the dungeon is BIG. The dungeon should cause players should feel that they are far from the lighted world above, especially as they moved deeper into the underworld.