The best feedback is inquisitive, not prescriptive.
Critiques aren't easy, unless you're versed in what makes them successful. It's a difficult task to focus entirely on the end user without personal bias. It takes restraint to move the discussion forward without getting caught up in sidebar discussions and taking off on a different tangent. Take a lesson from the pros at Facebook as they give an inside scoop on their process and it is evident that they are on top of their game.
Take a peek for yourself in the post by Tanner Christensen and you can learn a thing or two about the process and how to contribute.
Or don't and just hand out your opinion without being asked. Just remember that opinions are like assholes.