bmn : As far as I know, this is not possible from a GreaseMonkey script due to security restrictions. Alternatives include rewrtiing this as an extension, or using a pseudo-protocol to open the file with a download manager. I'm working on a more generic script supporting other websites and implementing the second option, I'll post it on userscripts.org soon.

Sven: yes, that's a pretty good idea. I have this working for redirector (e.g. asx) files, but it requires first downloading the file, then modifying it, thus more complicated with media files since I need a callback from the download manager to notify that the donwload is finished... With an extension that uses the built-in download manager this wouldn't be hard, but since it doesn't support stream downloading that's not really an option... I'll see what I can do.

Alright, I updated the script with a fix, thanks for reporting.

Um, probably a bug if the description is so short there is no collapse button? I'm affraid I've not been doing much YouTubing lately, could you post an URL where it does this so I can fix it? Alternatively, try setting:
var hideCollapse = false;

in configuration. If that doesn't help, also:
var expandDescription = false;

Updated script:
-Added a workaround for a bug where comment form doesn't get focus on click if it was focused before focusing the video (which prevents typing until you focused something else and refocused the text box...)
-Allowed the video to be 23 pixels larger on "fill" or "max" when limited horizontally.
-Added an option to remove description and tags collapse links and options to disable removing the quicklist intro and sp offers.
-Improved the display of retrieved comments and added a safety check to retrieval so partial comments aren't removed if download fails for some reason.

Nothing really essential, but you might as well update while you are here ^^

I want to parse comment for links to work around YouTube's stupid no external links policy, but I don't want to risk breaking anything with an overly complicated regex, any suggestion as to which pattern(s) I should look for? Replacing a dot with (dot) seems to be most common, should I go for just that?

Work "OK"? Meh ^^ Glad you appreciate. Tell me if you find any bug or have suggestions for improvement.

Thanks! I'm the author of this script but I forgot my pw and the feature to get it back doesn't work right now. Anyways I made another version with extra features, you may want to check it out:

http://userscripts.org/scripts/show/5906