XP for treasure, rumor tables, random encounters, ten minute turns, henchmen and hirelings, morale, loyalty, encounter reactions, random NPC personalities, alignment mapping, level titles, strongholds, sages, ships and naval combat, weapon speed factors, weapon vs. armor adjustments, spell casting times, bardiches, broadswords, spetums, voulges, resurrection survival chance, system shock, bend bars/lift gates, percentile strength, chance to know spell, inexplicably flammable lamp oil, item saving throws, diseases and parasites, rot grubs, banks and moneychangers, wills and inheritance, racial preferences, classes that balance mechanical uberness with alignment restrictions, AD&D-style multiclassing and dualclassing, barbarians that don't rage (berserkers go berserk, not barbarians!)
But on a more general playstyle level, I would say the essential aspects are:
Sword & sorcery
Sandbox
Players just say what they do and the DM decides how to handle it