Level drain.
More seriously, there's a few things that could make any D&D feel more like 1e:
- pull some of the rules back behind the DM screen: magic item lists and pricing, combat bonus by level (BAB in 3e), etc.
- no healing without use of either a divine spell, a potion, a device (rare!), or lengthy rest; and no magical healing at any range greater than 'touch'
- magic items have to save or be destroyed if carried by someone who fails a save vs. area damage e.g. fireball, lightning bolt, dragon breath, etc.
- an overriding sense, emphasized by the DM at session 0, that the game world will be out to kill the PCs dead; that bad things can and will happen to PCs so don't get mad when they do
- an overriding sense, borne out in play, that the party is more important than any one character: characters come and go while the party is (one hopes!) everlasting
- fewer mechanics - no feats, no skills except for thieving abilities, etc.
- no ASIs. Permanent stat boosts in 1e were very uncommon.
- harsher revival options after death including a resurrection survival roll to come back at all and permanent loss of a con point if you do
- henches and hirelings in the party; and multiple PCs per player whether played all at once or not