OD&D keeps characters simple. They don't have loads of spells, abilities, or magic items. The monsters are built in a similar way. An orc swings its sword or fires its bow at you, and that's about it. Critters like beholders and dragons are a little more complex, but they're the exception, not the norm. There are no skills to roll, just descriptions of what a character tries to do.
When you pull those things back, you're left with only one option for making a dungeon or adventure interesting: Compelling locations, mysteries, puzzles, weird phenomena, *stuff* that the PCs can poke, prod, and inspect. These are all the things that make D&D compelling. They show off the spontaneity, immersion, and creativity that arise in the exchange among players and DM.
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