It says in the New Forum Guide that links are generally postable and that the system will automagically™ try to parse them into hyperlinks, but only at higher trust levels. It also says you can put single “tick marks” (which I believe to mean apostrophes) around a link and it should display as text, if you’re not authorized to post links:
Maybe it’s just that you can only post links to the “approved” websites until higher trust levels?
EDIT: Well, apparently either I’m at a higher trust level than I thought I’d be (maybe they were gathering data from our previous forum usage to have a baseline when the new forums went live?) or the “approved website list” idea is a solid one, because even though I put apostrophes on either side of that YouTube link it works. 