Speaking of how play experience would vary depending on which rules you ignored, there were actually attacks of opportunity in 1e. They weren't called that, but the 3e rules largely codified and simplified the disparate mentions of the idea in the various places in the rules. In particular, you could not just "leave melee" in 1e. If you were in base to base contact with a foe - in melee with them - then if you tried to disengage from melee (break contact) you drew a free attack from every figure you were in contact with. And it wasn't just "an attack", it was an attack on the rear of your figure, which meant you were flat footed, couldn't use a shield, and the opponent got a +4 to hit you. Nor was there a 5' step in 1e, so breaking out of melee in 1e was painful to the point of being something you just did not do.