The website of Ten Ton Hammer has released a gameplay preview of Death Knight.

The details are as below:

Starting out
You start out at the top of Acherus, the Ebon Hold. It is a necropolis-like building. The first thing you see in the center of the platform is the Lich King himself, with your first Quest ready to be accepted. Alongside him there are two Valkyrie-like monsters that look really impressive, I assume will be monsters in Northrend you will have to fight as they were part of the Val'kyr.

The Lich King and his Valkyries 

The Lich King welcomes you and asks you to go report in to one of his subordinates in the floors below, who will teach you to forge your own Runeblade. With this Quest, you will be able to craft your first weapon and learn Runeforging, an ability that will let you enhance your own items later.

The first thing I noticed was the fact you are given a full set of green gear that's very decent for a level 55 character, and will only be upgraded as you continue completing Quests for the Lich King and his commanders.

Upon crafting your sword, you are sent out to battle with the Scarlet Crusade's forces through another series of Quests.

Combat
Now begins the fun part. The combat system reminds me a lot of how Warrior plays, with a little bit of a Rogue for finishers. Let me explain. You have 6 runes at your disposal, 2 of each kind (at least at the start), and a bar that's empty before you begin fighting. As you use your spells and special moves, your Runic Power bar goes up (like rage) and you exhaust runes (which have a ~10 second recharge timer). As you keep fighting, you may choose to unleash your more potent move (in the case of the Starting Area, Death Coil), which will consume your Runic Power.

There are many ways of using this system, and there are many different abilities with different rune requirements for you to use and optimize your battle rotations. This is a great thing for people who like variety, since if you want to tank or DPS, you can switch abilities accordingly.

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Resource: Ten Ton Hammer