One of the major influences on my gaming life is the Hero System. Just ask anyone who's ever gamed with me!
I've tried to introduce it to some of the folks I've gamed with here is because I really enjoy the philosophy underlying the Hero System - a core system with optional and alternative rules that allow you to model the genre that you want. I believe it also allows you to separate the setting from the ruleset, but allows you to see how the rules support the faux reality you're trying to achieve in the game, and really lets you understand how many of the rules you're using can actually be extraneous from the current game your running.
Every time I run a game in it, I learn something new in that arena.
However, there are some limitations that come from a relatively rules-heavy background. I feel that it gets you used to a scaffolding that can make you feel like rules-light games are a little more arbitrary than is comfortable.