Lookz, I has wicked copy and paste skills... I'm pasting this here because I doubt anyone will read the WoLK FAQ already in place
NEW CLASS: THE DEATH KNIGHT
Naturally, people are pretty worked up about the Death Knight being released. However, many don't seem to understand exactly how unlocking it is going to work. This is what Blizzard has said so far:
In order to unlock the Death Knight, you must perform a series of quests similar to the Onyxia key chain or the Warlock mount quest. This chain will begin somewhere between level 55 and level 70, as Blizzard doesn't want people rushing to level 80 purely so they can play as a Death Knight. When you have completed these quests, you will be able to create a Death Knight character that is pre-leveled to at least level 50 (exact value not confirmed). Your old character WILL NOT be consumed, or augmented, or anything like that. See the section on the Death Knight here to see Blizzard's confirmation of this.
The Death Knight is a tank hybrid, similar to Paladin in that it is a tanking caster, but different in that it focuses on dark damaging spells whereas the Paladin is focused more on holy and healing aspects. The Death Knight utilizes a new "Rune" system, different from Rage, Mana, and Energy. "When you are out of combat, you can switch your active runes around a few types; you have 6 rune slots. Abilities will put certain rune types on cooldowns or may even use multiple runes. Thus, what runes you choose affect how your deathknight plays in combat." There are three rune types: Blood, unholy, and frost. Information found here.
Death Knights will not use shields, but can dual wield as well as use the standard two-handed weapons. They can wear plate armor.
Also, Death Knights ARE NOT superior to the classes already in place. They are different, but not better.
So far, all races will be able to be a Death Knight, though this is subject to change.
Rumor has it that you might not have to be level 80 to unlock the Death Knight... this is still up in the air.