I'm a big fan of mplayerplug-in (and mplayer).
On the other hand I'm no big fan of flash.
With greasemonkey it's possible to watch the youtube videos with
mplayerplug-in. There might be other means to do this...
Just in case someone is interested.
Here's my setup:
1) Newest firefox from ubuntu edgy
2) Newest greasemonkey
3) This greasemonkey script: http://userscripts.org/scripts/show/6278