With the release of the WoTLK in mid November 2008, a new class, The Death Knight, was introduced.
The Death Knights are WoW's first Hero Class. What this 'Hero Class' exactly means is unknown because they have not proven to be very different then other classes except for the facts that you need to have a level 55 or higher level character to create a Death Knight, that the Death Knight will start as a level 55 upon being created and that the Death Knight uses Runic Power to be able to use his abilities.
I will not take away the fun by explaining the quest chain but I can tell you that the quest chain is cool and rewarding. You can reach level 58 in just an hour or two by doing the quests around The Ebon Hold so even if you do not intend to play a Death Knight it is nice to at least pick up some cool lore. Once you hit 58 you are able to go to the Outlands though and you know that the leveling there isn't time-consuming, so who knows; even if you didn't plan to play a Death Knight, you might still get stuck playing one because it is a cool class.