So, I just got a new Logitech C910 web cam, and when trying to join or create google hangouts, I couldn't see myself in the stream at all, nor anyone else's. I can hear everyone, and everyone can see me, but I can see nobody, nor myself. I found this gem of a site with a very easy solution.
I'm using gentoo Linux, and this simple trick made it all work for me.
The trick came from this site
The trick is simple: Issue this command, and voila! It works.
If your webcam lights up fine, you can hear everyone, but you can't see webcam streams, I have the solution for you. After installing the Google Talk plugin, run the following to add an override for your video settings.
sudo echo "O3D_OVERRIDE_RENDER_MODE=2D" >> /opt/google/talkplugin/envvars
Use sudo if necessary. Restart your browser after doing this and check to see if you can see video streams now.