I've talked about him before) said it was because of a GURPS game we played long ago, and the game's encouragement to select disadvantages and quirks. In GURPS, it's almost necessary to do so to get more points for character building, but as it turns out, it's also good at training players to gives their characters personality.
Let's look instead at most people's gateway into the hobby: Dungeons & Dragons. There have been various versions of this game over the years (and another is forthcoming), but D&D is NOT the kind of game where you pick character traits and quirks. Instead, the focus is on race and profession, frameworks that supply a variety of specific abilities. In the game itself, you are rewarded with leveling (absent in GURPS) and an increase in those abilities. Our question last Sunday was "How does that specifically train a player to approach role-playing in general?" Certainly, it creates the expectation of character evolution (perhaps as opposed to character development, which is a function of plot or subplot, not of ability growth). Classes provide definite roles for the players to fall into and encourage players to find and stick to a niche. D&D probably also promotes mathematical gaming where stats are more important than personality. But I don't want to get into hot water with D&D players just because I don't happen to like the game anymore.
Truth be told, the aforementioned Pout didn't start on GURPS - it wasn't even the first game we played together - so your "schooling" isn't limited to your first game, far from it. And just because you play a particular game doesn't mean you're limited by the way it teaches role-playing. There are plenty of D&D players who run deep, immersive games where personality and character (read: emotional) growth are at the forefront and not in the background, just as you can have running a looting-heavy game of Doctor Who RPG. It's just that each of those games doesn't MECHANICALLY pull focus to those aspects of gaming. And the mechanics of a game are very important in how they mold their respective players. By habit or necessity, you will progressively lean towards what the game asks of you, and then carry those attitudes to other, later games.
So for example, if games have some kind of Cinematic Point mechanic (Bennies, Hero Points, Karma, Star Power, etc.), it teaches players they can take an active hand in changing their environment, reverse bad dice rolls, etc. Therein lies the way to what some call "New School". Games with few stats and/or catch-all, open traits like Fate does, likewise, and put the focus on creating an original character concept. Traveler's character creation uses an unsual method, that of crafting the traveler's pre-game career, with skills emanating from that career path. That teaches players to give their characters a backstory. They weren't born on the day of the first gaming session. There are games where life is cheap and death comes all to easily, others where the characters are superhuman or protected by an aura of narrative importance. Sometimes, there's no limit to the kind of character you can create, and other times, the choices are quite limited. Each of these has something to teach, and depending on when you experience them, and whether that experience is positive or negative, it will influence you as a gamer.
So what was MY school? Well, we didn't have D&D books available in my town, so we house-ruled everything for the first 2-3 years until we did get our hands on the real stuff. And I can see that in everything I do. I've added hundreds of options to Dream Park, created a meta-setting for GURPS, tried out a few house rules on DC Heroes, streamlined AD&D magic and XP to cut down on bookkeeping, and created new abilities for Hong Kong Action Theater. I love published games, but I can't help it, I've got to add to them, tweak them to my tastes, and indeed, to reflect everything I've learned from OTHER games.
What about you? What lessons did you take away from your most formative role-playing experiences?