A rare example of Blizzard delivering faster than promised: They promised us that the Deathknight would be terrifying, and quite a lot of players of warriors and bloggers are already terrified! Via Random Battle I found this excellent analysis of Deathknights from Wolfshead. It is pretty negative, but well worth reading. Read both Wolfhead's original post and Cameron's comments and then come back here for my take on the subject.
At the root of the problem is a mismatch of the current reality of tanking classes and the promises Blizzard made about Deathknights being dps tanks. If we assume that Blizzard holds their promise and makes the Deathknight a class that can tank well enough in most group situations while still doing enough dps to be good for soloing and PvP, there is an obvious problem for the other tanking classes, especially warriors, for who this is not the case. The other possibility is that Blizzard doesn't hold their promise, and as Wolfshead suggests pulls a bait-and-switch job on us. You can see how neither alternative is very promising.
Playing a warrior I obviously don't want a new class in the game, played by a large number of players because it is the first new class since 2004, which takes exactly the same role that I have, only doing it better, and which rolls for exactly the same loot, except for shields. If that happens, I can kiss my warrior goodbye. But on the other hand I have a vested interest in Wrath of the Lich King being a big success, I don't want my already low-population server to become even emptier, so I don't want people to be totally disappointed by the Deathknight class either. And I think Blizzard is aware of these two problems, and wants to avoid them. But how?
One alternative is following a classic MMO class design rule, which says that a hybrid needs to be weaker in all of the roles he can perform than the specialized class which can perform only one role. Thus a Deathknight would be a less good tank than a warrior, and a less good melee dps than a rogue. Unfortunately that approach has two problems: it isn't very attractive, and on the tanking side it is very hard to design "less good". Damage is easily scaleable, tanking is not. Either you hold the aggro or you don't. If you scale down tanking so the Deathknight needs a bit more time alone in combat before the other players can start dealing damage, and then need to tone down their dps to not out-aggro the Deathknight, the average player will just declare the Deathknight being useless for tanking. You can already imagine groups of three Deathknights spamming the trade channel for a healer and a tank in that case, because everyone will think of them as dps classes. And a bit further down the road you'll see groups that still need a dps class advertising "need dps, no Deathknights please", because they prefer other classes with higher damage or more crowd control or other useful abilities. This is what Wolfshead describes, a solution where from initial design or with nerfs the Deathknights ends up being less popular than other classes, after the inevitable first surge.
A better alternative would be to make Deathknights as good as tanks as tank-spec druids and paladins are now. That is perfectly viable for most group situations, even including some raid encounters. There is a certain logic behind having all tank hybrids be equally strong. But to make this work, Blizzard would need to do a major rework of the warrior class, significantly improving the viability of the warrior tank in solo and PvP situations. And I'm not talking about minor improvements, as announced in patch 2.4, like Cleave not breaking sheeps any more. I'm talking about changes where a tank warrior would solo grind mobs at the same speed as a feral druid or Deathknight, and where a warrior with a shield is a serious threat to some classes in a PvP situation and not a cause for laughter. The 4 tanking classes would basically be equivalent in tanking, with each class having advantages in some situations over the others; and they would be equally good in soloing and PvP, just using different styles. One could for example imagine tank warriors shining in PvP with various shield maneuvers, for example being dangerous to casters with a much improved shield reflection skill. Basically the warrior would turn from being a specialized tank class to one of four equally good but different tank hybrid classes, and that without being penalized by having to pay dearly every time he wants to switch roles.
So this is where we are: We still have very little information about WotLK, and what we know is that introducing the Deathknight is not without risks for the overall class balance in World of Warcraft. You can take the alarmist point of view like in the quoted articles and be terrified of the Deathknight. Or you can hope that Blizzard takes the opportunity and creates a class balance that is even better than before, by upgrading the warrior and removing many of the problems that protection spec warriors already have before the Deathknight class is there. Does it make sense to have ONE dedicated tank class which isn't much use for anything else when chosing the tank spec? For a game that stopped being all about PvE groups, I think not. We don't need "group only" classes or talent trees in World of Warcraft. Let's all be hybrids!