With Wrath of the Lich King looming on the (far) horizon, it turns out that Blizzard may actually care what you want in it! They've just opened feedback threads for every class on the official forums, asking players to list one or two sentences about their top three problem areas, and their top five problem talents/spells/abilities for their class, along with what talent trees you tend to invest in and what sort of content you play most (PvP, raid, solo, etc.). It's a highly structured format, and you're also instructed not to engage in discussion with other posters or put suggestions on how things should be improved. Just point out what you have a problem with.
The posts note that when they feel they have collected enough feedback, the threads will be closed, so make sure you get your opinion in before that happens. Here are the links for the various classes on the North American forums: Druid, Hunter, Mage, Paladin, Priest, Rogue, Shaman, Warlock, Warrior. And here are the European links: Druid, Hunter, Mage, Paladin, Priest, Rogue, Shaman, Warlock, Warrior. It's possible that all the players speaking their minds have brought down the forum servers, since I can't get them to load, but I'm sure they'll be back presently. And you may not be allowed to have discussion in the Blizzard threads, but feel free to use the comments on this post: what's wrong with your class? What do you think they should do to fix it?