I reckon folks are tossing out the word "innovation" too lightly.
I reckon that the word "innovation" is simply not quite what the OP had in mind. The definition of innovate pretty much comes down to ANYTHING new or different. Pick something, pick ANYTHING, that wasn't in a previously published RPG. That is by definition an innovation. People aren't using the word too lightly - the word itself is too light for what I
think the OP was searching for.
That aside, the problem for me is that few changes, if any, have been particularly revolutionary as opposed to merely evolutionary. Oh you can pick many RPG's that made big changes - but they were changes largely limited to THAT RPG and not so much broadly influential upon RPG's in general. A classless RPG may have been
innovative but it wasn't particularly revolutionary - the bulk of RPG's didn't suddenly shift to a classless approach. It just became one more option among many for how to approach an RPG and hopefully it's recognized that while innovative it's just not an option that fits for ALL RPG's. The utilization of a more unified resolution mechanic is certainly innovative whether it be by a d20, percentiles, or fists full of d6's. But RPG's didn't then suddenly shift to overwhelming use of unified resolution mechanics when that appeared. Again I should hope that wise game designers understand that one mechanic doesn't always provide the resolution that fits all circumstances.
Now I haven't given this that much thought but at this point I can only point to two "innovations" that I think really substantially and broadly changed RPG's as opposed to simply adding one more to the myriad of existing options.
First, as has been mentioned already, is the advent of the internet. As noted it
isn't an innovention directly connected to RPG rules but it should be credited as such because of it's effects. RPG's were an extremely insular activity (that is to say,
much moreso than they still are today). Change spread by literal physical word of mouth as much as any other means until the internet began to provide a fast, even instant means of communicating ideas. Before then you MIGHT attend a convention or send letters to a magazine and read its articles, but that was about the limits of common communication among gamers in general. Innovations were predominantly found only within a few gaming groups. The number of people communicating regularly outside their own groups was an extremely low percentage of an already sparse community. When internet communication came along with dial-up BBS's, Usenet, and finally web-based message boards the PACE of innovation increased just as rapidly. There were a lot of innovative ideas out there - they just hadn't had opportunity to spread anywhere with any speed. It was the internet that revolutionized RPGs by simply allowing OTHER innovations to spread and take root. It further supports innovation today by providing the fastest, cheapest means of publication of new materials.
The second innovation is also something of a meta-innovation though it at least is directly tied to RPG rules. Of course it's the OGL. It's one thing for players to exchange ideas with each other but quite something else to promote the exchange of ideas between PUBLISHERS. It is apparant to me that while TSR was still in business they were unable to grasp the revolution that was the internet. While understandable, it was definitely misguided of them to actively quash not only innovation by other publishers for D&D but innovation by PLAYERS. They labored under what had become an OUTDATED philosophy.
Innovation and communication of ideas now had the perfect outlet and chafed while being contained or controlled by old methods. The OGL took the chains off and allowed the wider innovation to actually proceed that had been wanting desperately to take place for D&D itself. Of course that innovation didn't then limit itself to D&D but made possible innovations of altogether new RPG's that were based upon the successful D&D model.
