Chris,
The biggest single difference will be the revision of the PA/RF board in the 100D. You can study the differences for yourself by looking at the schematics:
viewtopic.php?f=35&t=2337&p=2342The older Rev. 15 and 16 boards actually had more signal routing flexibility. However, they also had poor crosstalk performance on the board which tended to limit the amount of PureSignal linearization performance that could be achieved. Many people, including myself, modified their units to add an external switch such that PureSignal feedback was applied directly to the Angelia RX1 input during TX and did not pass through the PA/RF board.
Neither revision is truly a bad performer, but people like to squeeze all the performance out of them that they can.
The only other thing that comes to mind is that the older units used to come with an internal linear power supply board. It was electrically very quiet but it ran very hot. Fixing the cooling problems on the 100, 100D and 200D's is easy: just put a small fan on top of the unit and make the feet a little higher for better air flow underneath. See here for more info:
http://wu2o.dyndns.org/wu2o_fans.htmlIt is possible to find gray-faced units with switching power supplies and Rev. 24 PA/RF boards, so you will have to inspect the unit carefully, perhaps disassemble it, to really be sure about what you have.
73,
Scott