And here most of us were, thinking this was going to end up
undocumented :P

A lot of people figured out the comment links, but didn't know that it
was possible to create URL's for video descriptions and external
sites.

As for the links in comments, I was only able to get it to work by
clicking the 'Post a Text Comment' link and posting the timecode in
that box, instead of the one that is usually visible below the
comments.  Not sure if that's how it's supposed to work or not.