> I'm looking for constructive feedback on what can make this a go-to replacement for awesome sites like jsFiddle.net and jsbin.com

This seems like the wrong goal. Nobody wants a replacement for an existing site, just because.

To quote Ashton Kutcher: Mark Zuckerberg will always be a better Mark Zuckerberg than you. You won't make a better jsFiddle by trying to be a jsFiddle.

Do something that jsFiddle.net and jsbin.com don't do, or do it much better (read: simpler). Quick idea: git access.