> Except that I don't have to understand the underlying data structure to use a more basic VCS like Mercurial. What makes git so special that I would have to do that before being able to use it?

I don't think it is special. Generally after a while using a given tool, library, etc. I find it useful to dig in a bit and see what's happening under the hood to help understand why it works the way it does. git just happens to be the tool under discussion at the moment.

> And for recovery from mistakes, I meant stashing the changes somewhere, deleting the repository and downloading a clean copy to start again, which I had to do a few times with Git and never with Mercurial (I might had to do it once or twice with SVN, though).

I think we're talking about the same sort of mistakes. It's hard for me to imagine a case where you'd need to blow away a local git repository entirely. Worst case scenario, there should be good refs available in a remote that are just a 'git fetch' away. (If there's no remote, then blowing away the local repo is essentially just starting from scratch anyway.)