Death Knights are unholy warriors who fill both a DPS and tanking role. This means they know how to deal the damage and take it. They cannot use shields making them the only tanks in the game who tank with two-handed weapons. Most of their damage mitigation is from their tough armor, their presences (see below for more information), and various abilities.

Pros on playing a Death Knight:

Death Knights wear Plate armor, start at level 55, and come with a set of uncommon gear and get a set of rare gear from their starting zone quests.
Death Knights use runic power and a rune system which is unique to the class and provides a new and interesting style of gameplay.
Death Knights can tank with two-handed weapons and get free weapon enchants as part of their "Runeforging" ability.
Resilient class with lots of magical abilities and melee abilities providing a dynamic combat style.
Can pull enemies directly to them with an instant ability.
Lots of abilities to take down casters.
Cons of playing a Death Knight:

Requires a character to reach level fifty five to unlock, meaning you cannot begin the game as a Death Knight.
Identity Crisis: Death Knights start at level fifty five, making them attractive for everyone's first choice when they decide to make an alt.
Cannot use a shield when tanking, meaning that damage mitigation is from the presence and abilities.
Starting quest can be annoying if you just want to hop directly out into the world.