I normally withhold my praise from D20 products, and it looks like the upcoming D&D 4th edition will have me laughing my ass off since they reportedly White Wolf-ed it out, thus bringing an end to the reign of the pure class-level systems. (Feeling slightly cynical, perhaps his death has something to do with what the hell they did to his game.)
But never let it be said that I refuse to give credit where it is due. Gary showed us the way to better tabletop gaming, and his system stayed the course a long time, long after many dissolved to mushy-ruled mishmashes or simply faded away. There are other schemes to how to do things. Some of them actually work, too. (Pity the rulesets tend to murder them.) But D&D was the first that gave you something both easily playable and that you could sink your teeth into. It wasn't the best, but the job of the pioneer is to put things on the map, not to survey them, so it didn't have to be.