Accessibility is a close cousin to usability. Instruction, no matter how amazing it might be, is of little value if the learner can not access it.
So, what are the big barriers of accessibility. The traditional set of barriers are physical limitations such as blindness or hearing loss. Addressing these requires both intent and specialized knowledge.
Once again, feedback is important. A development community will almost never have complete coverage of the spectrum of hardware and software in use. Systems and combinations where things misbehave are important to know. Further, accessibility can be a strong differentiator in online reviews of content.
There are some great resources for implementing and evaluating accessibility.
Shameless plug, I'd love to find someone to work with the combination of 3D modeling and 3D printing to provide greater accessibility to the visually impaired. Putting a well designed model in someone's hands will be a far richer and superior experience than a textual description.