To summarize, the OGL allowed anyone to distribute the core mechanics of the D&D rules. It allowed anyone to write & publish adventures compatible with D&D, and modify & expand the rules however they imagined. There were specific comparisons to the GPL used in open-source software (something also close to my heart, since I've been a professional software developer). The business case was that WOTC made money selling hardcover rulebooks, while others could write adventures that supported the system. There was a "d20 System" trademark made available, for free just by sending a letter to WOTC, that allowed anyone to signal compatibility with the greater system of D&D.
It might help to refresh one's memory with how Ryan Dancey was selling the OGL, back at the time of 3E's rollout in 2000, on the WOTC site here.)
I previously wrote about how I saw the breakup of the OGL happening at the advent of 3.5. The evangelizing Ryan Dancey had left the company at that point, and WOTC was pretty clearly stepping back from the OGL movement. They didn't communicate the rules changes to the companies that were dependent on the system, and many were forced out of business at the time. (You can see my prior posting on the 3.5 release here.)
I think the switch to 4E is going to be significantly more dramatic. For starters, there is no longer a free "d20 System" trademark. WOTC will be offering that to only a few select, approved large companies, and perhaps approved sub-licensors. They claim that there will be an OGL version of the rules, but details are extremely vague (frankly, I remain skeptical at this time). Third-party publishers seem to be suffering even more greatly this cycle; apparently no draft of the rules has been shown to third-party publishers (whereas they did have a draft at the equivalent time in advance of 3E). Some folks are reporting local game stores getting out of the D&D business entirely with the rules switchover. The head of ENWorld reports that their publishing sales have fallen 80% since the announcement of 4E (here).
But by far, the greatest failure has been in the promise that the OGL would make the future evolution of D&D a cooperative effort between company & customers. Here's Ryan Dancey, from the above article, as 3E was being rolled out:
The other great effect of Open Gaming should be a rapid, constant improvement in the quality of the rules. With lots of people able to work on them in public, problems with math, with ease of use, of variance from standard forms, etc. should all be improved over time. The great thing about Open Gaming is that it is interactive -- someone figures out a way to make something work better, and everyone who uses that part of the rules is free to incorporate it into their products. Including us. So D&D as a game should benefit from the shared development of all the people who work on the Open Gaming derivative of D&D.
It was precisely this promise that was the biggest attraction for me to get back into 3E. Like the open software movement, the intellectual property was being opened to everyone. The users themselves would be driving the future "shared development". For me, I expected this to result in smaller changes over time, and an ongoing refinement of the system in the future, as consensus grew around particular parts of the ruleset. That's a big part of why I thought it was worthwhile to learn the new system, that the rate-of-change would be slowing, and therefore it would get wider usage for a longer period in the future.
That promise doesn't look remotely like the releases for 3.5 or 4E. In fact, it's pretty much the exact opposite of "someone figur[ing] out a way to make something work better, and everyone who uses that part of the rules... incorporat[ing] it into their products. Including us." Rather, 3.5 and 4E changes have come entirely from "star" designers in the WOTC offices. The new changes have in fact been closely-guarded secrets, sprung on the industry in the most surprising way possible. Hints and clues about the new 4E system are locked behind Gleemax logins, or even sold as "design preview" books in game stores (with the mechanics details still secret).
Clearly WOTC isn't taking direction from the larger community... in fact they apparently feel compelled to make as many surprising mechanical changes possible, so as to startle the players and get attention with the widespread, fundamental novelty. My estimation now is the reverse of what I thought when 3E was rolled out -- instead of changes slowing over time, I now expect them to accelerate over time into the future.
Consider Ryan Dancey's later analysis of how the the OGL played out to 2007 (a month before 4E was announced), in response to a question of mine posed at ENWorld here:
In practice I'm not too surprised at the lack of OGC use in D&D. There are several dozen people paid quite well to design & develop Dungeons & Dragons. You will remember that just after 3.0 shipped, Wizards went through 4 disastrous rounds of layoffs. The "survivors" (many of who also survived the Last Days of TSR) know how to keep their jobs: Do great work, and get it published. So there's an economic (and personal) motive to avoid using a lot of 3rd party OGC in the work that team produces. Nobody in that team wants an upper level manager to think "well hell! Let's just get rid of the RPG R&D group and rely on this talented pool of freelancers instead".
I'll also say that there's a unique thing going on at Wizards of the Coast, and that thing is what I call the "culture of design"... The result is, in my opinion, a qualitative difference between the WotC produced content and a lot of 3rd party content... it's hard for outside material to make it through the design inspection and review process and into a Dungeons & Dragons product.
So there you have it. The initial promise of the OGL (in any way that resembles the GPL of open source software) is basically pronounced dead by its initiator (and also a current WOTC spokeperson in the same thread). Refinements produced outside WOTC can't become part of core D&D products, because (a) the mind-set is now that WOTC provides a "cathedral" of unmatched game design high priests, and (b) the game designers themselves need to convince their bosses that only their work passes muster (and so specifically avoid OGL sources). And I'd also suggest that (c) WOTC wants to "suprise" the player base with many unexpected changes in order to drive sales. Whatever form the OGL'd content for 4E takes, the motivation behind the original idea is definitely no longer there.
Thus, for me, one of the biggest promises of 3E, which brought me back to the game, has specifically been reversed and renounced. If the players aren't formally participating in the evolution of D&D, then it just looks like any other RPG product to me. If there isn't the promise of everyone have the opportunity to publish work marked as compatible with D&D, then it's not so exciting. If it looks like changes will come more and more rapidly in the future, then it's not worth my time learning a new system today.
But, we'll always have 1E and 3E. Maybe if the pendulum swings back in the right direction for 5E, I'll once again take a look. Until then, cheers!