I did this pole a while ago:
http://www.enworld.org/forum/d-d-4th-edition-rules/205162-what-must-d-d-have-giant-poll.html
Actually before 4E, but from it, 4E has these things in common with past editions. Though some parts have changed:
Dice: d20s, d4s, d6s, d8s, d10s, d12s, d100s
Abilities: the classic 6, ranging from 3-18, or (or 19 or 20)
Alignment: chaos, law, evil, good
Races, both in general and specifically: human, dwarf, elf, variant elf, halfling, gnome
Classes, both in general and specifically: : fighter, rogue/thief , wizard/magic-user, cleric, ranger, paladin, barbarian, druid, bard
Character level and XP (for defeating monsters)
HP & AC (as damage avoidance)
“Fire and forget” magic (including more explicit vancian magic for wizards)
Mechanics for listning, sneaking, finding traps, and climbing walls.
Many (statistically) different kinds of armor and weapons
Gems, gold (coins), and magic items
Dungeon, Wilderness, and (maybe) city adventuring
Monsters: dragons, orcs, kobolds, goblins (and kin), giants, trolls, beholders, mind-flayers, drow, giant/dire insects and animals, elementals (and kin), demons and devils, fey, griffons and minators (and so on), dinosaurs, tarasque, evil humans
Whats interesting is that there are plenty of RPGs that have few of things, and or deemphasize them. But there are also plenty of RPGs that have most of these.
But how many have ALL of them? And wouldn't that be a D&D clone?