One nice thing you can do on Vimeo (at least with an upgraded account) is swap the video file without changing the url. It's quite handy if you are in a rush to post something, and then make some minor tweaks later.
As mentioned previously, the password protected features are great, but youtube is catching up with the private and unlisted.
I use both for work...hate youtube, but the audience is there....love vimeo (as mentioned previously: compression is better, no ads, less crap, dynamic community, decent (but not great) support)
cb