Why might a 1994 Bonneville stop moving at a traffic light where the engine revved but the car would not move?

If it's automatic, check the fluid. Low fluid will do that.

I had a similar problem, the car was very slow to start moving. The cause was the transmission range switch was loose and out of adjustment. Another way to tell if this is the problem is to turn the ignition switch on and put it in reverse. If the back up lights do not come on and the bulbs are good, then this could be your problem also. Hope that helps!