Here's something that I have been curious about - the biggest perceived disadvantage of git is that it is not written in a single language - this leads to the problem that it is more difficult to set up on Windows than mercurial.

Isn't this a problem that is easily solved ? I mean Github is a pretty large company built on top of git. I know that Github is sponsoring work on libgit2, but JGit is pretty good as well (and so is the pure python implementation - Dulwich).

Would it cost too much for Github to sponsor a bounty to make a pure python/ruby/java based implementation and keep it in sync with Junio's git ?

Though it is not the intention, I'm pretty sure if you had a 100% Github-supported Dulwich git... it would be very hard for mercurial to stand up as an alternative. Which is why it is hard for me to understand why isn't such an obvious business lead being taken advantage of. I genuinely want to understand whether it is a very hard technical problem - which will make me respect Linus even more.