Slower character progression. Though I like the simplicity of the current experience mechanic, I don't like the effects of it. Characters gain levels too quickly for players to master them. And by using a one-size-fits-all xp advancement chart, many classes are stretched/limited to keep them "balanced".
Named demons/devils. Asmodeus, Baalzebul, Demogorgon, Dispater, Jubiilex, etc. Had many great adventures based around these guys.
Artifacts. Artifacts in 1E had certain base abilities, then a ton of random abilities - just rolling those abilities would give me so many plot ideas. Also, there were a
lot more of them, with some interesting back-stories.
Magic items not limited to spells. "Protection from..." scrolls. Potions of Giant Strength (rather than bull's strength), Dragon Control (vs. charm monster), Invulnerability (vs. ??). Staff of Striking. Staff of Withering. Staff of the Serpent. You get the idea...items designed around a theme, not a spell.
Flavor. I know, this doesn't fit the original question, but it's still a valid response, IMO.
Size. PHB was 128 pages. DMG was 240. MM was 112. Total: 480 pages, roughly half the 3.x total.