It's \\\ and it comes from the commentary plugin. It was the default mapping when it was released but it was changed a few months ago. \\\ is still available, though.
Yes, lots of people use <leader>something mappings because they offer a lot more flexibility than <C-something> and friends. What you use as mapleader is up to you, "," is simply the value suggested in the doc.
I keep `q` to record macros, but complement that with :noremap <Space> @q That way I can press `qq` to start recording, `q` to end, then just wack the spacebar to execute. I use it constantly.
comma is really useful as a leader and it's not so far away on the keyboard. No one I have heard of uses ctrl for leader(it would ruin a lot of ctrl-whatever commands) and by default the leader is a backslash.
Yes, lots of people use <leader>something mappings because they offer a lot more flexibility than <C-something> and friends. What you use as mapleader is up to you, "," is simply the value suggested in the doc.