There are two kinds of accessibility that come up in gaming discussions. One is that understanding how a game works, or how to master it, can often be impenetrable to the newbie. But the other is that no matter how experienced a gamer is, if he or she has certain disabilities, the games can be literally almost impossible to access.
Features like subtitles, control remapping, and color-blind modes are a start, but many games still don't take physical disabilities into account. A new project hopes to increase awareness about how and why accessibility considerations can add to a gamer's experience and increase the audience.
Called Game Accessibility Guidelines, the project separates adaptations into a list of basic, intermediate, and advanced steps a developer can take to make sure their game is accessible. Basic features include common features like control remapping, separate volume controls for speech/music/etc, subtitles, and high contrast on text. The advanced list includes features like a "pingable sonar-style audio map," distinct audio cues for different events, and allowing all instructional and narrative sequences to be replayed.
The full list is exhaustive, and particularly worth a look for able-bodied players and developers. It's easy to take something like audio cues or even having two hands to use a controller with for granted. And while some gamers will always get hung up on feeling that mastery of a complex system is the winning skill that sets them apart, others would just like to be able to adjust the controls so they can play at all.
As for why accessibility is important? Other than the important issue of basic fairness and decency, the project points to data PopCap gathered showing that up to 20% of the player base for casual games have some kind of disability. That's a lot of gamers who are currently being underserved—gamers who could be buying many more games, if developers take their needs into account.