I've been kind of uncomfortable with Pathfinder and D&D both for different reasons, and started looking for another role-playing game that has the rules for free online and lets you write your own stuff for it.
Both are strongly inspired by Pathfinder and D&D, with stock fantasy adventuring tropes and more or less stock fantasy character options. But the authors went in two different directions with them ... especially with regard to how accessible their games are to newbies. Whether those newbs are players, or fan / professional authors.
Here's the intro to Dungeon World, heavily excerpted to show you the main points.
What is Dungeon World?
Dungeon World is a world of fantastic adventure. [...]
Why Play Dungeon World?
First, to see the characters do amazing things. [...]
Second, to see them struggle together. [...]
Third, because the world still has so many places to explore. [...]
How to Use This Site
This site is going to teach you how to play Dungeon World. [...]
Each of those ellipsis shows where they went into more detail about something, for about a paragraph. We're just showing the main points here, because we feel like, this is the best and most concise introduction to an RPG ever. It's a game, it's awesome, here's how to play. And what's especially fantastic is that it does that without directly invoking Dungeons and Dragons, Pathfinder, or any other RPGs.
This game is an entry point to the roleplaying hobby.
Here's the 13th Age SRD, for contrast:
What is 13th Age?
13th Age is an "open" d20-based tabletop fantasy RPG similar in play to games like D&D 3.0/3.5 and Pathfinder.
What makes 13th Age different?
13th Age makes use of many game mechanics and features that are intended to develop characters and story as the game is played. For example:
And then it goes into detail about its unique game mechanics. Don't expect the official 13th Age website to give a much better intro; that link takes you to their blog. You have to click on "13th Age Core Rulebook" in the sidebar, to see more about the one product they most want you to buy. And that takes you to this page, which spends more time talking about the "legendary" authors and quoting people who liked the game than actually explaining to you what it is.
The 13th Age website's FAQ explicitly says that "The [13th Age core] book assumes some amount of experience with RPGs, and in particular d20-rolling RPGs." They needn't have bothered. No one is ever going to look at this page who isn't already experienced.
Whether or not any newbs ever see Dungeon World is another matter. But if they do, like on the shelves at a local games store, they won't spend long guessing what it is or figuring out who it's for. Even the name, cover art, and shape of the logo, besides evoking the D&D brand and tropes, immediately convey what the game is.
13th Age? The back cover has an endorsement where one of the Penny Arcade artists facetiously says that he played a lot of "12th Age" and loved it, and this book is totally an improvement. How's anyone supposed to get the joke without knowing a lot about tabletop RPGs (starting with the fact that the "12th Age" game doesn't exist)?
Even the name, 13th Age, doesn't hint at what the game's about. It sounds kind of like "Dragon Age," but the connection is tenuous at best. You have to either be very intrinsically motivated to find out what this book is for, or be in the kind of social circles where people are already talking about it.
Some Rights Reserved
Finally, the last thing we wanted to compare was their licensing model, which is relevant to us since we write RPG stuff.
Dungeon World is licensed CC-By. You can do anything you want with it, so long as you credit them. They even have a handy logo that you can use.
13th Age is licensed under the OGL. That means putting a copy of the OGL itself in your game or add-on, and listing everything that your work derives from all the way back to D&D 3.5, as well as specifying what you're releasing as Open Gaming Content and what is Product Identity.
But while you have to credit them in the license itself, you can't say that your stuff is compatible with 13th Age or its "Archmage Engine," or use either of their logos to promote your work. You have to be someone they consider a "professional publisher," and then apply for the rights to use either logo. Which is basically how it works with Pathfinder, too.
I can't tell you how much I hope to see games and creators move away from this rent-seeking behaviour. And how much it tickles us to see both Fate and Dungeon World "licensing" work the way we feel most people would intuit, without reading legal text ... give the creators credit, don't claim to be working for them, and use the free logo they give you.