MMO Blogger Lume the Mad has a nice in-depth review of all the changes coming to World of Warcraft in the form of the game's second expansion, Wrath of the Lich King. If you're a regular reader of WoW Insider, there's nothing really new here -- odds are you already know about all the changes (and have a strong opinion of some of them). But as a nice wide general overview of what's going on with the game, Lume's little writeup is very thorough.
It is pretty amazing to think that before Burning Crusade released, there were no such things as Blood Elves or Draenei, the only Arena in the game was Gurubashi, and only the Horde ever had Shamans in their raids. But nowadays all of those things are commonplace, and odds are that the game will change just as much with Wrath -- siege vehicles will be a huge part of the game, Death Knights will be everywhere, achievements will be just as big a goal as any other part of the game, and phasing will be the norm (you'll never be sure that the player next to you is seeing exactly what you're seeing in the game world).
Lume doesn't go so far as to call it a new game (though Blizzard may want to), but there's no question that even though the most basic game mechanics stay in place (you still use abilities on action bars to kill creatures and gain experience), the quests, graphics, and storylines will look very different. BC changed our game once, and Wrath is set to change it yet again.