The forums are a good tool for developers but it is not necessarily the best or most efficient way to give and receive feedback.
For one, forum opinion may or may not necessarily reflect the opinion of the playerbase as a whole. I have observed many gameplay changes that have been widely praised/condemned on the forums but experience conflicting or completely opposite opinions in game, within my guild, and within my server community. This does not happen all of the time, but it does happen often enough that I have learned to distinguish from "forum opinion" and "opinion from players I meet in game and talk to".
So what would be a better way to give feedback? In my opinion, in-game surveys that are account-based.
Why surveys?
1. They are accessible to everyone with an active account. Due to a myriad of reasons, not all players can or choose to post on the official forums to voice their opinions (ahem, players with "forum bans" or players who simply do not utilize the forums).
2. Surveys allows players to "vote" once per active account on features they would like to see implemented. No more players jumping on alts to voice their opinion for or against a given feature in forum threads!
3. Surveys tied to accounts allows developers a way to analyze a lot of player opinion data very quickly (who does and doesn't like x feature, their pve/pvp participation, subscription history, etc). This way, Blizzard can see what type of player likes which particular features, and can adjust accordingly.
To encourage survey participation they can award some sort of in game perk, like a unique pet, etc.
For players who have left the game, surveys could be given through battle.net or via email.
Feel free to give your feedback :)