I've been thinking, with all the discussion of what goes into a campaign, about the variants I encountered in the early days of the hobby. First of all, it should be said that folks didn't talk about playing D&D so much as they spoke of playing in Cameron's Shattered Lands Campaign. I think that speaks to the mindset of the rules being a starting point and not some sort of goal to be acquired.
That aside? The first variant I encountered was not a critical hit/miss system or spell points. Those are the ones that I always hear "everyone used." Oddly enough, it would be a long while, almost 10 years, before I encountered either of those in play (and I've never used either so you know at least one old school referee who didn't). It was a home-grown called God Rating (GR).
GR was basically roll high on d20. If you beat a target number you rolled again, receiving a little extra perk for your character. I seem to recall I got another d8 hit points. I've long forgotten the name of the referee who came up with the rule, I only played in the campaign for a short while, but I recall the rule. I played a Ranger in that game, I remember the PC because it was my first 18 Strength fighter-type. He was slain in his first combat by a wild boar but fortunately I was playing 2 player-characters and just kept on going.