I’m going to throw an idea out there, and that is that the single, most important contribution that WordPress has made to open source software as a whole is documentation.
When I first started using WordPress 8 or so years ago, that was the biggest difference between WordPress and other platforms. You could search for something and actually find the answer. There was even a huge wiki dedicated to how to use — and modify — the platform: the Codex. With other open source web application software platforms at the time, documentation was always scarce. The first Magento project I worked on, I had to teach myself how their theming system worked. Likewise for ZenCart and Joomla!. This self-education takes time, and this is the whole reason we say “I am a WordPress developer” as opposed to “I am a web developer.” Sure, I have skills that extend beyond WordPress, but I know WordPress in a way that I don’t know other platforms. I am much more able to work on the fly on something I’ve never tried before in WordPress than I am on a roll-your-own platform or some other CMS. And the availability of documentation plays a huge role in this.
The two WordPress-specific businesses I’ve worked for — Event Espresso and WebDevStudios — both have had their own, internal documentation based (at least in part) on the WordPress Codex. That’s in addition to the user documentation that’s readily available for most premium plugins. The docs may not always be complete — and they may not always be good — but they are there and you can usually find answers. Plugin developers specifically are motivated to provide good documentation to eliminate the amount of support requests they get via support forums. These support forums are a form of documentation, too. I asked a question on a Magento forum a few years ago and I don’t think I ever got a satisfactory answer back. If that happened on a WordPress forum, the hounds of hell would be unleashed on the plugin author or, at the very least, everyone would start to avoid that plugin. If it was a WordPress core component, a Trac ticket would crop up pretty quickly with a long discussion about how best to solve the problem and, eventually, a fix would get built into WordPress core.
WordPress people are always talking, always communicating, and this is part of what helps WordPress grow. The first blog platform I used was sBlog, which had little-to-no documentation and a very small community around it. If you’ve never heard of it before, that’s because it doesn’t exist anymore. A slightly better platform I played with for a couple years (which does still exist) is Ampache, a web-based music player where a lot of discussion and documentation happened in the forums or else in the IRC channel on Freenode. But because there aren’t blog posts about “I built this awesome thing with Ampache” — and because there isn’t the amount of documentation for Ampache to help developers build awesome things — not many people know it exists.
And that’s part of the reason why I was initially lured into the Docs contributor team. WordPress documentation is a huge part of how I got where I am today, it’s what sets WordPress apart and helps it grow, and it’s vitally important to the continued success and growth of the software. But what I’ve noticed in those years since first toying with Joomla, Magento ZenCart, sBlog, and Ampache, is that other projects now have more documentation available, and put more of an emphasis on documentation. Look at HTML5 Boilerplate or Bootstrap. Look at Git and jQuery. Spend some time on StackExchange. There are tons of answers out there now, answers that weren’t there for us 8 years ago. I feel like the success of WordPress has brought with it the rise of better documentation — and those platforms that fail at documentation get passed over by ones that have documentation. And that documentation increases in relevance and quality as things like MediaWiki have cropped up and allowed for the crowdsourcing of said documentation so that anyone can be an editor or an author of a tutorial or code reference. Yes, I credit at least some of this to the popularity and rise of WordPress as a publishing platform, and I suggest that it is the most valuable contribution by WordPress to open source software. Even if WordPress someday fades, its’ footprint will be left by the emphasis on — and prevalence of — good documentation.
Is it possible that my glasses are rose-tinted because of my involvement in the WordPress community? Sure. But the thing is, WordPress is the most common CMS in the world. More than 20% of the web is built on WordPress and the percentage of new sites using WordPress is even higher. So, one way or another, other projects will need to learn by the example set by WordPress if they want to stick it out and become a viable platform that people can pick up and use without having to dig through lines of code to figure out how.