The new Help Center could really use a comment formatting article of some sort.
While I do know about http://meta.stackoverflow.com/editing-help#comment-formatting at the moment, I will eventually forget about it. Once that happens and I have a question about comment formatting, I will try going to the help center, and searching for "comment formatting" or perhaps "comments", hoping to find a link to such a reference.
At the moment I would be utterly disappointed. There does exist a help link for comments, but the fact that I have never noticed it before, means I won't see it once I forget about it again.
I'd suggest: an article that mentioned the basic italics, bold, code, and links syntax, and the mere existance of @username and short hands links, and which contains a link to editing help page for full details.