Challenge, resource management, tier expectations, and art.
Challenge used to be a cornerstone of the game. Low hit points, dead at zero, properly scary monsters, negative conditions that last, and slow healing.
Resources management used to be a cornerstone of the game. Tracking food, water, light sources, weight carried, ammo, etc. It gave a real sense of grounding in the fiction and world.
Tier expectations used to actually change the way the game was played. You'd go from delving dungeons to exploring wilds to leading armies, to leading kingdoms, to questing for immortality, to actually being an immortal.
The art used to be way more evocative. Characters were shown in fun, funny, silly, and scary positions. Black and white line art filled the pages and kept the costs down.