My concern is not that we aren't getting new races. I am worried that adding only one class will toss things out of whack. When TBC was announced, they let us know that the Horde would be able to play paladins, and so for balance they added shaman to the Alliance side. Balance is the key word here. When we have Death Knights added to the game, there will be only one Hero Class, albeit available to both factions. I get the feeling that this will throw off the balance more than they realize. Not so much because there will be another tanking class, but more because Blizzard cannot anticipate how players will actually play the character. Sure it's a tanking class, but if players start using it as a DPS class despite the best intentions of the devs, things could go badly.
Considering the magic-centric storylines in Northrend, I would feel more comfortable if they added the Archmage along with the Death Knight. Adding the Archmage would allow players to play a mage of any race, opening up a whole new world of role play possibilities. Or how about the Demon Hunter? With so much going on in Outland, couldn't we use an anti-Illidan to clean house, so-to-speak? What do you think? Is Blizzard making a mistake by adding only one class into the game?