Threading - anything but French. You also want to avoid JIS sized headsets, too - they come with a 27.0mm crown race/30.0 mm OD, but in Campy/Stronglight, etc, you'll not find JIS anyway. Italian threading differs only slightly (angle of thread cut) from English, but the thread size and pitch are the same so English will work on an Italian threaded steerer. But like Cyclotoine said, stack height is going to be your biggest issue.