I came across this video made by someone. I apologize if it's been posted already, I haven't read through the entire thread.
http://www.youtube.com/watch?v=qDLuKfMt8jA&feature=endscreen&NR=1
It's seriously the most brilliant* SFM video I've seen. I've watched it four times by now, and giggled out loud every time.
It's not nearly as advanced as some other SFM videos out there, and the part right at the beginning felt pretty pointless, but the it's an amazing example of the principle 'less is more'. Keep that in mind, guys. If you don't have the time and patience to make an epic sketch with Valve-quality body animations and a continuously clever plot, you can just make a video of silent, subtle face posing and it may end up being just as entertaining.
If you feel like doing something like that, try throwing in some grunts, sighs and/or laughs as well. I think those kinds of noises could improve it even more, and there are certainly a lot of great ones to pick from among the classes' voice responses.
* I haven't watched a lot of SFM videos.