Wrath of the Lich King is still in beta, and the level cap is still only 77, but the Death Knight class is a hot topic of discussion among World of Warcraft players. They follow unique mechanics unlike those of any other class, using runic power to fuel their eldritch wrath.
The Death Knight is a hero class - any player with at least one level 55 character on any server can create one, which starts at level 55 in a special instance that introduces them to their character. The Death Knight wears plate armor, can summon Undead minions to do their bidding, can tank, deal damage, drain life and put powerful debuffs on their enemies.
Death Knights dual-wield and use two-handed weapons; using their unique runeforging system to augment the various abilities that they use. The Death Knight is an interesting class in that their talent trees don't determine their class role so much as how they accomplish what they want to do. Every talent tree allows a Death Knight to both tank and do damage, although there are some talents that are good for either. Blood focuses more on physical damage dealing and health regeneration, frost more on debuffing and physical mitigation, and shadow more on spell damage mitigation. In turn, each tree has its own damage-dealing style.
No Death Knight will be the same, but the focus on their overall role in the group will remain constant.
It's an interesting setup for a class, and so far it is working quite well. However, until the beta matures more and the level cap is raised to 80, it won't be clear as to whether or not a clear "best" specialization will arise - and Blizzard doesn't want it to. They've managed to achieve a pretty solid balance between the other tanks in the game already - each has its strengths and weaknesses - so it's not completely outside the realm of possibility that they'll be able to accomplish that with Death Knights.
The runic power system allows the Death Knight to choose an arrangement of six runes before each battle, and each of their abilities uses a different number and combination of runes. This makes it so that a Death Knight has to plan out their moves before every battle, and encourages everyone to develop their own style that fits well with their specialization. Runic power generates through the use of certain abilities, so it's a bit like a combination between a Rogue's energy and a Warrior's rage.