"The following equipment is required to be operating prior to entering RVSM airspace:
• Two independent primary altimeter systems
• One automatic altitude hold system
• One altitude alerting device
• Transponder (altitude reporting)"

This is cut and pasted right out of our ops manual. And, though it's not listed above, the MEL does say no RVSM if the TCAS is inop. Most RVSM aircraft are going to have two transponders, though. You could "lose one", turn the other one on, and still been okay for RVSM.