What aspect of RPGs has had the biggest effect on you?
The fact that I can't do it alone. I spent fourth through seventh grades at a high-dollar private school, on a faculty scholarship. I didn't fit in with those kids, who were from a different world of money and privilege. Like they say, the schools may require uniforms, but the kids still know rich from poor.
Nonetheless, after four years, I'd adjusted pretty well. And right about that time, my dad changed jobs, the scholarship went away, and I was suddenly back in public school for eighth grade. Not the best year of my life, I can tell you that much.
The following year, I discovered my high school had a wargaming club. While we weren't playing RPGs yet, it was my first real exposure to a peer group that shared my interests (and economic status) in many years. As RPGs supplanted wargames, I found myself making friends, growing more confident, and much happier.
Throughout my life, my RPG hobby has been my one constant social outlet. Even my other hobbies (comics, SCA) grew out of interests I developed and people I met via gaming. Without it, I don't know who I'd be, but I suspect that person would be a good deal lonelier.