Sacred Cows for any edition

[Maldin]

I actually use nine ability scores (Comeliness, Perception, and Willpower), so I obviously don't consider the six standards alone as sacred, but those six do have to be there. Hit Points and Armor Class are key concepts, but they don't have to be called that (Health and Defense, anyone?). Classes are also key (although, again, name changes are ok, e.g. Thief = Rogue). Levels and Races are also a must. And the mechanics of play which many people have already listed... such as rolling dice for attack, saving throws, spellcasting rules roughly what they are now, etc.

Denis, aka "Maldin"
Maldin's Greyhawk http://melkot.com
Loads of edition-independent Greyhawk goodness... mysteries, maps, magic, mechanics, and more!