Using capacitors for ethernet isolation?
The Ethernet transformer, whether a separate component or integrated within a "mag jack" allows for complete isolation between the power and ground domain of the Ethernet controller and the cabling domain. Same thing on the other end of the cable. 

Using capacitor coupling will typically mean that there is some GND sharing between subsystems or ends of the Ethernet cabling. Unfortunately the pinning of an Ethernet RJ45 jack does not have provision for a common GND connection across the Ethernet cable.