So just what is a hero class? While you might think that means it's a more powerful class, instead it's a more unique class -- or at least it was at the time it was introduced in the Wrath of the Lich King expansion (and you'll need the expansion to play one). Today, some of the unique characteristics of the class mirror changes we've seen come to other classes, but the story of the death knight class -- which ties closely into the story of Arthas and the entire Wrath expansion -- is still a unique experience you won't find anywhere else in the game. That's not to put down the class: just don't expect it to be more powerful than any other class.
There is one newbie-unfriendly caveat: to create a death knight, you have to have another character at level 55 or higher, because death knights start the game at level 55. It doesn't matter what server your level 55 character is on or what faction it is, but you have to have one somewhere before you can create a death knight. This isn't to say that new players can't tackle death knights -- but you'll have to spend your first 55 levels playing another class.
Sound interesting? We'll break down just what death knights are -- and aren't -- below.