Originally Posted by EvanM

3. Unknown - Player/NPC's labeled as "Unknown"

This issue is named after the names other player have when one is experiencing this problem. The WoW client on your computer is not receiving a name for the other players so it names them, by default, to be "Unknown". Other symptoms include players appearing without armor as well as your character model appearing plain white when you view your character sheet (C key).

This problem stems from the information that is coming to your computer from the realms being blocked or filtered in some way. There are 3 main causes to this problem listed below in order of likelihood.

# Checksum offload setting
# Duplex setting
# Router firmware

Please review the instructions here for the checksum offload and duplex settings first:
http://www.blizzard.com/support/wow/?id=aww01686p3

You may need to contact the router's manufacturer for directions on checking and updating the firmware for the router. An easy step to check if the router is causing the information to be blocked in this fashion is to remove the router entirely. If connecting the computer to the modem resolves the problem then the router is blocking or filtering the information in some way. Remember that you will have to turn the computer and modem off before removing the router.