There are a few topics for which a FAQ would be very useful. I'd suggest that the Twin Paradox would be a prime contender for its own FAQ entry with at least two explanations - ie a (visual) geometric explanation and a (mathematical) relativity of simultaneity explanation.

Another contender for my money would be why you can't get a velocity from "contracted length / dilated time" (and how length contraction and time dilation differ from Lorentz transformations). Similarly a FAQ entry on what "coordinate time" and "proper time" mean, along with their distance equivalents.

Another option would be to open up sub-forums for these hot-button issues. That would mean that rather than have twins discussed over and over again, because the earlier threads are not easily seen, a new arrival to the Special and General Relativity forums would see there is a specific area for "twin paradox discussions", and one for "time dilation and length contraction", and so on. Each of these could have their own FAQ.

I can't remember all the recurring themes off the top of my head, but I am sure some of the other regulars can.

I always like http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html" [Broken] for the twin paradox which points to the FAQ and a set of experimental tests verifying the SR calculation.