Well... poo. :)
I don't know what to tell you then. Make them into mimes?

Actually, I think that's all they have for range of facial expressions. D:
In Half Life 2, it wasn't necessary for these characters to have advanced facial expressions. Therefore, I don't think faceposing for these characters is included in their rig for SFM. Tell me if I'm wrong, but I think that's the problem. Nothing you can fix unfortunately, unless you make your own rig and faceposing for the character, which is a complicated process I am still trying to figure out.

Glad to hear that the img sequence rendered fine! :D Though if I were you, I would still try to figure out what went wrong so that you can avoid it in the future.

Are they AMD? Their system of updating is a little weird. Here's their support site.
http://support.amd.com/us/Pages/AMDSupportHub.aspx
If the problem persists, I would as a last resort reinstall SFM. Just make sure you back up your files first.
Also another thing to try is taking an old video you made that worked and re-render it and see if you have the same problem. If you don't, then you know something's gone wack in this particular video and not in SFM itself.

It shouldn't take longer to render (I would think it would take less time, but I'm not sure). One thing I recommended to another person having rendering issues is to update your graphics drivers if you haven't already. I had that issue a while back so it might be worth looking into.

Good start! You have some storytelling going on, but it is a little unclear in some spots as to what exactly is happening. I would take a look at this tutorial that Zachariah put up in regards to the line of action: https://www.youtube.com/watch?v=j-Uf-lsH_AQ
That should help you get clearer storytelling with your shots.
As for animation, the more you practice the better you get. One thing I think you should work on in particular is speeding everything up. I know you were trying to sync your animation to the music, but the slow motion is just not working well and honestly makes things a little boring. A lot of beginner 3D animators (including myself) tend to create animations that are too slow. Timing is half the battle with animation. So if something is too slow or too fast, you're either going to bore people to tears or confuse the hell out of them. So just keep yourself in-check when animating a long video like this.
Hope this helps and keep it up! :D

Yeah, I'm using Maya to rig and animate the characters. I just need to get the Valve plugin installed. I'm meeting up with my map maker in the next couple of days, so I'll be sure to relay that information to her. Were both pretty pressed for time on this, so the easier the better. That's neat about the high poly models stuff, too. Didn't know about that. Thanks, Gamerman!

Have you updated your graphics drivers? I may just be blabbling my head off here, but that was an issue for me before, especially if I had multiple programs open at once. If you haven't updated them in a while, might be worth seeing if that works.

That is a HUGE help, walropodes! Thank you very much!!!

Yes I heard about that. We were going to try Wallworm once we get a block-in to test with. Hope it works! D:
The other thing I am worried about is that one of my characters (the one I am working on now) has an advanced rig, so I can't use the standard Valve biped. I think I found what I need in terms of scripting on the Valve Developer Community site, but if you guys have any advice regarding that issue, it would be much appreciated.