I realize your druid argument was really about PvP but I am going to talk about PvE here. The problem with ferals in BC was that with a single spec they could do nearly as much dps as a rogue and also tank as well as a warrior. The warriors and rogues understandably cried foul. If you look at the feral tree for Lich King, you'll see that it does have more bear-centric and cat-centric talents. If you pick the cat-centric talents, you'll still be able to tank to some extent, but you probably aren't your guild's main tank. Likewise, there are definitely talents that will help a DK be a good MT. Blade Barrier is a great example. But there are other talents too. You need some of them. You don't need all of them. We don't want either the druid or the death knight to be jack of all trades, master of none. If you want to tank, it's going to come at the expense of some dps talents. But you'll have more options about which tanking talents and which dps talents you do take. The trees are designed so that you can't have them all.