Takralus mentions the three C's of feedback - be clear, concise and constructive. What does that mean?
First off, identify what you are talking about. "I am having trouble getting into PvE" or "I feel that there is a barrier to entry in Arena PvP" for example tells people what your issue is. This also feeds into the concise portion of the advice above - brevity may or may not be the source of wit, but on the forums it's very helpful. Don't take six paragraphs to say what you can say in one. And yes, I am aware of the irony that I, of all people, are telling you to keep it short. But less really is more in the world of feedback, with CM's trying to read through hundreds upon hundreds of posts in a day so that they can give a report to the devs about what players are saying.
Finally, how do we manage 'constructive' criticism? Well, that's where we avoid certain traps and simply explain what we see as the problem and how it could potentially be solved. I want to emphasize the word potentially here. First, I'll use an example from my own case - I recently wrote a post discussing how the proposed level 100 talent Extreme Measures seems to fall a little flat, and suggested a way it could be improved. That suggestion is perfectly fine, but just because players suggest something, it doesn't follow that Blizzard will implement that suggestion. Some times, getting a lot of suggestions just shows the developers the pitfalls of common player thinking - a consensus of a lot of players can be utterly wrong, but it's still useful to see in what direction that wrong consensus is moving. What constructive criticism never entails is rage based invective, complaints like catering to the X or other tropes we've all seen. Constructive criticism is useful - it lays out the problem and either suggests ways to solve it or how to best identify it, and it moves on without ad hominem attacks or fevered rhetoric.
This doesn't and should never be taken to mean that Blizzard is going to immediately change the game based on player feedback, even if the development team recognizes that there is a problem. A lot of warrior players have been complaining about haste for years now, and Blizzard is well aware of the problem - but that doesn't mean they can just borrow any of our proposed solutions and slot them in. Balance in World of Warcraft is an extremely complicated system of interrelated parts at this point, after all. Many times, a solution to a problem is designed, implemented, and even works - but creates new problems. Look at the rise of the LFD and LFR tools, which have overall greatly benefited the game since they were designed and implemented - both have created entirely new issues.
Feedback is clearly important at Blizzard - looking at how Warlords of Draenor intends to handle endgame content, we see its evidence. Iterative design always finds a balance between what players say and do and the vision the developers have for the game. It's undervaluing the role feedback has to ask if it matters, but overvaluing it to become discouraged because not every suggestion on the forums gets a blue response or incorporated into the game.