I generally don't like to post negative questions, but I think we all have a story here. Player Mementomori of the Dethecus server kicked off the discussion by picking Desolace as the worst zone in the game. He cites a badly placed graveyard and lack of decent quests. Its only redeeming value is being of comparable level to Stranglethorn Vale, so it's a good place to avoid that overrun zone.
Popular vote getters are Un'Goro (which gets my vote with those damn dirty raptors plus being so far away from Alliance cities), Azshara and Swamp of Sorrows. Even a blue threw in his two cents. CM Drysc's vote went to Shimmering Flats. Great to look at, not much to do. In fact, that seems to be the theme of most of the complaints about zones: visually appealing, but lacks significant content.
What about you? What do you consider the worst zone in the game? And why?