Defining moment? I'd say it was somewhere between the time that my players started exploring the world outside the dungeon (AD&D1e, probably around 1980) and the time I played in the second big campaign I got involved in at college. Basically the first experience suggested the possibility of using an RPG to explore a coherent world based on real-world premodern models. I then set about trying to create a campaign on that basis, while also picking up bits & pieces of the same idea from Runequest 2 & Ysgarth (though I only ever got the corebooks of those games). I never ran that campaign but a friend ran something along similar lines from roughly 1984-86, which basically set the pattern for the kind of gaming I was most interested in--basically rather low-powered fantasy with a strong S&S flavor.