At their core, they both add a more intelligent layer to information and knowledge. Sort of like evolutions of Wikipedia, just forked in different directions; in 2013, it might be more fashionable to call them "knowledge graphs."
RapGenius started off with its namesake niche, but is slowly expanding into other topics; Quora began by allowing all the things, although its initial audience sort of funneled that "everything" into a handful of buckets. I think it's still an open question for both platforms as to if/how/when they'll explode out of their founding communities and become a mainstream "thing" for all types of knowledge.
Most of the content on Quora is original and created specifically for Quora's format, whether it's an answer, a blog post, or a review. RapGenius has historically been a layer on top of existing information (lyrics, books, speeches), but it can become an avenue for original content. Their famous post Heroku's Ugly Secret is not just a solid technical read but also a fantastic advertisement for how their platform can add value to vanilla writing.
Annotations are an unexplored, yet incredibly fitting, medium on the internet; I'm waiting on the first Big Thing where annotations play a creative role in how the content is consumed and interpreted. There are some really well designed blogging platforms coming out, but it's tough to say if any of them are bringing something as fundamentally fresh as annotations to the table.
And for fun, this Quora answer, annotated on RapGenius
Edit news description to add:
- Historical context: how the event or text affects the world and history
- An explanation of the work's overall story (example: "Here, President Obama confirms the legality of drone strikes...")
- The work's impact on current issues