You can now embed YouTube videos in discussion threads. To do so, put [media] before and [/media] after the YouTube URL for the video.
If I wanted to embed this video -- http://www.youtube.com/watch?v=NX9AjUxTPGY -- I would just put that full link between the [media] and [/media] tags in the main body of my post, which would then look something like this:
And result in the YouTube video being embedded like this:
If you want to do it "graphically", then you just click on the Media button (it is the button in the editor that looks like three pictures stacked on each other) and in the resulting "window", just type or paste in the full YouTube video link.