Limit Break Dancing
Regardless of where you stand in the whole "edition war," I think we all can admit that the 3rd Edition family of products was, and remains, highly successful...both from the financial perspective and from the hobby perspective. There are a lot of reasons for this, but I think that the biggest reason for the success of 3rd Edition is the Open Gaming License. Why, you ask? Well:
Commercial Availability of Add-On Products
In a nutshell, having an Open Gaming License meant that third-party publishers could more easily produce a larger variety of gaming products. This created a large amount of gaming material on the market: gods, weapons, spells, feats, classes, everything. If you were wanting a Viking-themed campaign setting in modern-day Scotland, with psionics and dinosaurs, you could cobble together everything you needed from the Core Rulebooks and two, maybe three, splatbooks.
True, the "purists" of the hobby would argue that this wasn't necessarily a good thing. Not all of the products that were released were of the same quality, and this might have been a problem for some people. For the first time, there was a "glut" of gaming material on the market. This was only a problem for Wizards of the Coast, who lost the ability to control the quality (and scope?) of their brand. Remember the OGL revision that was pushed through when "The Book of Erotic Fantasy" was released?
(Personally, I think it is impossible to have too many gaming materials, regardless of quality. To me, they are like bacon. "Oh no, people in the industry are writing too many game supplements!" sounds a lot like "Oh no, people in the kitchen are cooking too much bacon!" to me. But I digress.)
Creativity and Development
The 3rd Edition was more than just a canned roleplaying game. Thanks to the Open Gaming License and its d20 System Reference Document, the floodgates of creativity were opened and new spinoff games and rules were developed. This was more than just modular, plug-in stuff like classes and monsters...there were entire game worlds and sweeping changes to the rules system. "Tome of Battle: The Book of Nine Swords" laid the groundwork for the 4th Edition fighter, for example, and the d20 Modern game showed us just how versatile the d20 System Reference Document could be. Many of today's games, from the 4th Edition to the latest batch of kickstarter "retro clone" projects, owe a lot to the 3.5E SRD and OGL.
But again, many will say that this isn't necessarily a good thing either. By handing other publishers everything they needed to create our own rules and games, Wizards of the Coast created direct competition for themselves. But one has to wonder if this was the fault of the Open Gaming License, or if it was the fault of poor business decisions and plain old bad timing.
When WotC stopped supporting their line of 3.5E products and moved on to the 4th Edition, nearly every third-party publisher followed suit--except for (most notably) Paizo. This created the perfect storm: not every gamer wanted to switch to 4th Edition, after all, and now there were very few sources for the material they still wanted. They turned to Paizo, which grew fat from the sales and contributions of non-4E players. Four years later, they were the leading publishers for D&D-compatible material, and they continue to be today.
The 3rd Edition of the game, along with its Open Gaming License, celebrates its 12th birthday this year. And adventures, campaign settings, and other 100% compatible gaming materials are still being developed, written, published, and purchased for it...even more than 5 years after the 4th Edition was announced. We can't say the same for older editions of the game, and I don't think we will be able to say the same for the 4th Edition of the game. The OGL was more than a development tool; it was a life insurance policy for the brand.
Ease of Use
The Open Gaming License, together with its System Reference Document, were valuable tools for more than just third-party publishers. They gave average gamers like you and me all the tools we needed to build our own game worlds and house rules, and a means to distribute them across the internet with other gamers without any of the legal headaches that can sometimes arise in the Internet age. Now, instead of a three-ring binder full of photocopied notes, you can hand your players a thumb drive with your favorite HTML copy of the System Reference Document (edited just the way you want it). Or put it on your website. Or stream it via torrent. As long as you follow the guidelines in the OGL, everybody is happy.
Maybe Paizo would not have been as successful if WotC had continued supporting the 3.5E system (or at least continued to sell the Core Rulebooks and PDFs). Maybe if 4E had had a less-restrictive OGL, Paizo would not have enjoyed such a high demand for their product. Maybe 4E would have enjoyed a larger fan base if there were a larger variety of 4E-compatible products on the market. Maybe, maybe, maybe. We can only speculate, and your guess is as good as mine. But we all can agree on these two things: the 3.5E OGL helped Paizo, and the lack of one hurt 4E.
If 5E is going to do more than just "survive," if they truly want it to be the "edition to unite all gamers," then it absolutely must have an Open Gaming License...a real and robust one, not a weak and anemic imitation.