How I Would Do SEO for Rap Genius (2012)
moz.com
moz.com
The main takeaway for marketers who don't have the scale of content that RapGenius has should be this: content is king.
Long form content is back and making stuff people want to read is more important than ever. Great content + the semantic web is something everyone can leverage to increase their site visibility.
Your comment about storing each comment in a separate URL seems to be the true value here. Then each landing page can distinguish itself from the dozens of other lyric clone sites.
Disclaimer: used to work for Tunewiki, another lyric site