all in all, if it is a concern/suggestion for the game, there is a "Submit Suggestion" button in the Customer Support section of WoW (where you make ingame tickets). If it is about a ticket you received that you are giving feedback on the Game Master that addressed it, there's a survey at the end where you can write comments in the end. When it comes to suggestions and complaints, make sure you are backing up your concern with objective facts, not "i hate when, it sucks, this blows, this is stupid" statements. The more factual, objective information in your report, the better.
other option is general forums. There isn't an email for complaints though.