Have used Youtube for a while so wasn’t sure of what to post. First, I’ll start with embedding a video:
Mike Wesch produces a lot of excellent material on learning, technology and culture, which he uploads to YT. Which leads into the first tack that I’ll take: copyright. One should only use material from YT that the copyright holder has agreed to add to the site. Sometimes it’s not obvious (for example, though Mike is featured in the clip, does he hold copyright, or is it the institution at which he was speaking – I’m assuming the former).
It’s also important to note that YT (like any other website) has terms of service that one agrees to by the process of joining. Of particular interest is this one:
C. You agree not to access User Submissions (defined below) or YouTube Content through any technology or means other than the video playback pages of the Website itself, the YouTube Embeddable Player, or other explicitly authorized means YouTube may designate.
People often like to download them using converters such as Zamzar. The reasons peole feel the need to do so I suspect have to do with network (un)reliability, blocking, etc. If you really need to have access to a clip off-line, the thing to do is simply contact the copyright holder. They may be happy for you to do so!