Do a Google search for "world of warcraft port". Posts on the WoW user forum verifies without TCP port 3724 open, the game will not work. TCP 3724 for the game data, UDP 3724 for the voice. Did you try the rule above for blocking port 3724? In case it was not obvious, use the src-address for your local computer/network, and dst-port=3724. The src-port will probably not be 3724.
As a test, you might want to try this:
/ip firewall filter
add chain=forward action=add-src-to-address-list dst-port=3724 protocol=tcp address-list=WoW
Move this rule to the top. Then use WoW. See if anything ends up in the WoW list here
/ip firewall address-list
ADD: Do you have any entries in "/ip firewall mangle"?
Ok so good news the firewall is detecting this string and marking correctly. Based on this what rule would you use to drop it - because the one I've made isn't dropping the connection.
And yes I'm using a mangle rule on the LANs