Like others have said, the communities built around
3e has had a huge impact on the ability of DM's to improve their game. There's just so much material out there all centered around
3e. That has certainly helped my game, and I doubt I'm alone in that.
One thing that I think that d20 did was hurt, at least for a while, other game development. There was a fairly lengthy period there, while the bubble lasted and perhaps a bit longer, where every game under the sun flocked to the d20 system, whether or not it really fit. In the last couple of years, the indie press games have really skyrocketed in both quantity and quality, and I think that has to do with a number of people trying d20 and then realizing that, while a fantastic rules set, is not the best for every game.
One other area that d20 really opened up is the 3rd party sources. I mean, if you want to use D&D, you can find support for your playstyle, no matter what. There is at the very least, one sourcebook or advice book that will suit your tastes.
One thing that I do think has come out of
3e is players take a much stronger look at mechanics now. Every mechanic has been hashed out, rehashed and folded, spindled and mauled repeatedly. The transparency of the rules has made evaluating mechanics a more methodical process.
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