Below is a picture of my board:



-On the upper left there is the USB chip.
-Main circuit is powered by a 220/7.5 transformer
-USB side is powered by a 1W DC/DC converter (Output~5.5v)

Problems that I could spot myself:

1) The horizontal design where both USB and RS485 ports are side by side, should be replaced by a vertical design where ports are on the opposing edges

2) The isolation gap between two grounds is not big enough

3) Whole device can be powered from USB port, eliminating the EMI from power lines. (The reason that I didn't design it this way from beginning is that I wanted to isolate the PC from the converter as much as I can)

4) Place a ferrit-bead to filter power at USB chip VCC pin(s)

5) Reduce the copper in USB port area to the least possible area.

6) Place the USB chip inside a faraday cage?