Your response answers your question about Rap Genius.
It's so valuable because they figured out how to gain traction and build a large community, it's not a lot more complicated than that. Now they're going to attempt to leverage that into everything else that can be annotated. Combining all of that in one location = worth a few billion dollars potentially, at least in this inflated market.
They built a slightly better mouse trap, or at a minimum got the mice to come to their mouse trap instead of other mouse traps (most lyrics sites are terrible).
For most consumer sites, the technology is far easier than acquiring the users and a community that meets a certain quality standard. These days the technology part is really easy in fact.
People are telling you why your question is flawed from its premise without actually coming out and saying it. Try reading between the lines a little bit.
