Over at the always interesting -- except when they're arguing about politics, ugh -- Knights & Knaves Alehouse, there's a fun thread the offers up some excellent guidelines for designing dungeons à l'ancienne école. Most of them come from remarkable mind of the equally remarkable Trent Foster, with additions by many other fine individuals, including Matt Finch of Swords & Wizardry fame.
None of the elements mentioned below are part of a magic formula. You can include every single one of them and still fail to capture that elusive old school essence. Likewise, many adventures included none of these elements and yet their claim to the old school moniker is beyond dispute. As I say in the title of this entry, these are "guidelines," not requirements. Use them more as a tool for divining the nature of the old school mindset than as a checklist of things you must include in every adventure and you'll find them very useful indeed.
1. Environmental hazards -- slippery floors, rooms that flood, narrow ledges over steep drops, rooms that are excessively hot or cold, rooms or corridors filled with poison (or otherwise magical) gasses, etc.
2. Combat encounters should generally be with baseline (or near-baseline) monsters with difficulty enhanced by the circumstances of the encounter (i.e. monsters have set up ambushes, monsters forcing the PCs to fight in unfavorable surroundings, teams of similar (or dissimilar) monster-types working together, etc.) rather than through templates or class-leveling
3. At least one encounter that if played as a straight combat will totally overmatch the party, but which can be avoided or circumvented by some clever means
4. At least one puzzle, trick, or obstacle that requires the players to figure it out, rather than being solvable by a die-roll
5. At least one item, location, or creature that causes some kind of significant permanent effect (permanently raise/lower stats or hp, permanently change race, gender, or alignment, permanently grant or take away magic items, etc.) determined by a random roll on a table -- with possibilities for both good and bad effects, depending on the roll
6. At least one item of treasure that is cursed or has other detrimental side-effects on the owner/possessor
7. Some sort of "false climax" where inattentive players will think they've won the adventure and either let their guard down or go home, while clever players will realize this couldn't have really been the climax
8. At least one disorienting effect. teleporter, mirror trap, [swiveling] floor, or maze like monster. up is down too.
9. An area where resources are an issue. wet torches or wind blowing them out. oxygen low or having to hold your breath to swim [through] a tunnel.
10. An area that has items of value. but they are too large to transport. or cause someone to have his hands full at an ambush.
11. A creature that appears to be something it is not. Some examples: Lurker above, mimic, [cloaker], wolf in sheep's clothing, doppelganger, gas spore (perhaps my favorite), etc.
12. One encounter (no more, no less) that makes absolutely no logical sense, that the DM completely leaves up to the players' imagination to explain.