Fighter, Cleric, Mage, Thief only. All human. (Some grognards won't even use the thief.) You can play a nonhuman, but they find their level advancement strangely limited. (You can look up the level limits or roll d20 to underscore the arbitrariness of it.)
No death saves--0 hp and you die.
Spectres, vampires, etc. drain levels permanently. Poison is instantly lethal on a failed save. Petrification is instastone if the save fails, etc. However, the demons and devils, while capable of using lots of spells as at-will abilities, are referred to by strange nonsensical names with apostrophes in random places, have very few hitpoints, and suddenly fall dead in combat.
No caps on spell damage--that enemy 20th-level mage does 20d6 fire damage with a fireball. (Everyone wakes up alive after this, of course...)
Resting only recovers 1 hp per day--you have to cast and rememorize healing spells.
You stop getting extra hit dice after level 9 (you can fine-tune this if using AD&D rules).
There is a puzzle involving inserting the correct polearm into the correct hole.
The party's treasure disappears and is replaced by science-fiction stories nobody wants and a knockoff collectible card game.
There is insanely impractical skimpy armor.