thomas, I had a bad motor mount on my right side a few weeks ago. What I noticed was a bad vibration when stopped and the car was in drive. The vibration was very noticable... the whole car shook. When I took the car out of drive the vibration mostly went away. I bought 2 replacement mounts form Fastlane and replaced them myself. The old right mount was over an inch shorter than the new one! After I installed both the new mounts I started the car. I had to do a double take because I though the engine didn't catch... I gave it some gas and to my surprise the engine was infact running. I had gotten so use to feeling the car move when starting with the old mounts in. It took me a couple of days to get use to the vibration-free waits at stop lights... I had thought the car had stalled on a number of occasions.
I don't know specifically what driving around with a bad mount will do to your car in the long run, although I do recall a post I read in the archives that said, in short, the torque and movement of the engine will put unessasary strain on other components - exhaust, transmission and anything else connected to the engine.
I suggest replacing both mounts in the very near future