Ah, the d20 System. First introduced in 2000 in the third edition of Dungeons & Dragons, the d20 System marked the beginning of a new era...the age of unity, the age of universal mechanics. Yes, universal mechanics (like GURPS, or the Storyteller/World of Darkness d10 system) were not uncommon at all, but it wasn't until the 800-lb. gorilla called D&D started using the d20 that the idea caught on with "mainstream" role-playing.
It wasn't just the "brand" though, that made the d20 catch on. It was also the OGL, or "Open Gaming License." Basically, the rules for the d20 System were free to obtain and use in anything you wanted. That was a far rarer phenomenon (though again not the first; see the FUDGE system). This meant any Tom, Dick, & Harry Game Company could make a roleplaying game, use the d20 system, and make money. The players won, because they got fresh new content using a familiar mechanic, and the designers won because they didn't have to design, playtest, and analyze a whole new game system.
Flash-forward 13 years later. The d20 System is still around, and still powering some of the biggest names in the industry. D&D, now on its 4th edition, continues to use the d20 System, albeit in a slightly different form. Every other day, it seems DriveThruRPG.com has some new products that some garage designer made for d20. The market is flooded with d20 OGL stuff. And the fatigue has set in.
Nowadays, d20 has become a bit of a polarizing entity. Mention the system, and people will either think it's the best thing to ever happen to the hobby or the worst. Crazy thing is, they're both right. The hobby as it exists today was practically built on the back of the d20 System. It's led to stagnation in some, content to just keep cranking out books using the same old material, and innovation in others, looking to make a name for themselves as "Not That Other Game!"
I tend to find the best of all worlds within the middle of the two extremes. Systems that build on that solid d20 foundation, but verge off in meaningful ways carry a lot of appeal for me. They're familiar, but fresh and baggage-free. Notably, 13th Age and Numenera have both just come out, and they both have done this to a remarkable degree. Both systems are solidly d20-based, but both diverge towards a more free-flowing, narrative-oriented style that traditional d20 never catered all that much to.
Hardcore fans of narrative-heavy systems like FATE may scoff at the "innovations" that these two games bring to the table, but the subtle shift in design in these games should not be underestimated. Just the other day, a player in the D&D 4th edition game I'm running this Sunday sent me a 600 word write-up of her character's backstory. It was really great stuff, and the player definitely put some real thought into it. My first thought as I read it was "Whoa! This would gel beautifully with 13th Age's One Unique Thing concept!" As it is, though, we're playing D&D, and I'll be hard-pressed to make any of this backstory fit in anywhere meaningfully with the adventure as written.
Anyways, I don't mean to condemn D&D, or the traditional d20 System. Far from it. I just think the dynasty created by this system is extraordinary, and though it's easy to look at wild alternatives for change, I think it's the subtle changes that, in time, may be the most significant influence on the hobby's evolution.