A W5100 on board (soldered or not) would make the difference between arduino and this board. Assume that this chip driver will be integrated in a library for the FEZ. Everything on 1 pcb the size of the domino. I Never heard anybody complaining about the size of the arduino. If we could foresee PoE (Power over ethernet) by just using 4 wires of the utp cable.
=> The purpose is to get a system that has one connection to the other world for communication and power.
I believe that soldering is a problem for some people. Maybe you can offer the kit soldered, not or semi. In that case we save on the expensive (no chinese) hour rate of the people soldering.
I think :
domino size = arduino size
domino features (amount of io, spi, uart,…) >>>>>>>>>> arduino features
(domino + W5100) price <<<<< arduino + ethernet shield
Just my 5 cents.