Death Knights were built to be what Blizzard referred to at the time as a Heroic Class. They hit that ball out of the park. The DK straight out of the starting area all the way to level cap was and has been a true hero. When they were first released, it wasn't strange to see groups of four dks and one healer. The Death Knights were destroying the pve and pvp scenes like no other.

Anytime a class is extremely over powered from the other classes, lots of people will start to complain about it on the forum, and that's exactly what happened to Dks. Lots of their abilities have been reduced. But one thing remains evident, they are still a heroic class. Even players who have no idea how to play the class are doing insane amounts of dps. The Death Knight doesn't stop there. Their plate armor makes them a sufficient tank class as well. I've personally tanked with every class in the game, most all the way to level cap. Death Knight tanking is one of the most difficult, do mostly to the shortage of tanking abilities.

That shouldn't deter you from tanking with a Death Knight if that's your goal. They hold aggro with dps, so just be prepared to break faces. The most important thing to take into consideration as a tank isn't your spec, but your gear. Make sure while leveling you have as much dodge and defense gear as possible, and always wear it over dps pieces while tanking.

I know this because I leveled my death knight tanking dungeons before looking for group was around. I leveled mostly blood, and never had any major issues holding aggro, or getting healed.

I have to say of all the parts of the game I played with my Death Knight, pvp stands out as the most fun with this class. Heavy armor, and big burst dps make this a really fun class to play in pvp. Also the abilities the dk wield are perfect for almost every class in the game. Perfect for the Death Knight. Ranged, and other hard to defeat pvp classes have a hard time taking down a Death Knight, especially one who has the right knowledge to make it a very bad day for any other class.