A lot of time bending and refitting the GMT and hour hands to not hang up on the dial markers or each other, and a slight bend of the dial center. The problem is also compounded by getting the Rolex date wheel overlay to work properly and not hang up on the extra GMT setting wheel. I wouldn't recommend trying it and if you do, leave the crystal off until you get all the hands working properly so you don't have to keep taking it apart when they aren't set perfectly.
Now that I have it all working, I guess it was worth it, but I'd never do it again.