We all know by now that Death Knights are the new class being added with Wrath of the Lich King. And, what's more, they're the only class being added with the new expansion. Skudo (Altar of Storms) doesn't like that much, and would rather see Shadow Hunters or Demon Hunters added.
Posts like these are common enough, but Nethaera showed up to add some clarifying candlelight. First, she reiterated that Blizzard isn't closed to the idea of adding more classes in the future. But what she really seems to strive to get across is that adding a class isn't easy.
There's a lot that goes into creating a new archetype for players to operate, play, defeat, vanquish, and exploit for every bit of Warcraft goodness. There's balance to consider, and the need to make each class play in unique ways. Heck, we all spend enough time arguing now about whether the existing classes are even balanced. Can you imagine what it's going to be like with a brand new class added?
So these things take time. Assuming that Blizzard's team pulls off Death Knights well, I'm pretty sure we'll see more new classes in later expansions. Let's see how the first new class goes, before we put our eggs in new baskets.