To carry forward from 1e to 5e:
* Percentile dice rolls - far more granular than a d20. Use them for:
- Resurrection survival rolls: the risk, however slight, of having death be truly permanent (or at the very least a bloody great pain) is an excellent motivation to not die.
- System shock survival rolls: this was the great balancer to polymorph, that you might not survive the change. I've also found it a very useful mechanic in lots of other situations.
* Cleric-turning-undead chart: need to expand it a bit to allow for all the newer undead creatures, but the idea of "roll a d20 against this chart and see what you've done" still beats all the undead-turning systems since.
* High-risk high-reward magic: scry-buff-teleport loses much of its appeal when there's a risk you'll arrive 10' down in solid rock and immediately die. Fireball stops always being the answer when space is confined and it still wants to fill its alloted volume. And so on...
* Magic items that break, sometimes painfully, when mistreated.
* A sense that all rules are guidelines, and open to interpretation.
* Mechanics, as far as possible, kept behind the DM screen. BAB, magic item lists and prices, saving throws - let the DM worry about that stuff.
* Simplicity of character generation at low level and relative simplicity at high level: "character build" as a mechanical concept did not exist, a character was "built" through what it did in the game and how it was roleplayed in the doing of it.
* Wild magic.
* Reference to real-life pantheons, cultures, etc. I want to play a Thor Cleric, not a Priest of Belar(TM).
* Hirelings, henches, and associate adventurers.
* A design robust enough such that variable party size, and variable levels within the party, doesn't shatter it.
There's more, but that'll do for a start...
Lan-"oh, and donkeyhorses"-efan