If you have OCD.
Well I do, so that's a start. My first Death Knight was made with the help of my children. There was no way that I was going to create a DK without the input of my two boys. We debated for days, actually, trying to decide what race to choose. We knew that it had to be Alliance, all my main toons at the time were Alliance, so it only fit. We also agreed that it had to be a male, because I teach my kids that sexism is a good thing. (That last sentence is untrue, you know, a joke, just in case your jaw dropped too far). Actually we decided it would be a male because that was the next in line. I had created 1 male then 1 female then 1 male then 1 female, and so on and so forth, and so my DK had to be a male because that was the next one in the oder of things.
After about 4 days we finanlly decided to make a Night Elf. Firstly, I had no other NE toons, secondly, to be a master of Death and Decay with one who previously fought tooth and nail over the rights of tree spores and fruitlings seemed appropriately fitting. Having finally selected my male, NElf DK I entered the world of the DK for the first time.
The starting area questlines were astonishing. The story pulled me through and engulfed me round about. Every task had meaning, while providing learning and entertainment all along the way. I quickly grew into my first plate-wearing melee fighter and could feel the difference as I found myself well protected from outside forces. As I blew up an army that was clearly having a beach party, I was hooked. After leaving the starting area I quickly leveled up my DK to the point that he became my highest level toon, sitting at 72 and ready to push forward.
Then it happened...burnout. Ugh! Disease, Disease, attack, attack, death coil, dead, loot. The repitition burned me out big time. I fell back in love with my other toons, getting two of them to 80 before my DK ever sniffed 73. But a few months ago, after a year or so away, I went back to ol' NElf-boy and pushed him forward to 80. Then something really strange happened, I started to find playing a DK fun again.
DK's are unique, they can tank or dps with all three specs (for now at least). And all 3 specs, for however similar the opening two spells are, work very differently. I respecced my talents mabye 20 times, and enjoyed every single playstyle there was. Dual-wielding, 2H masher, Disease spreader, all of them held some appeal to me. I found that having a set rotation, unlike the priority system I was used to with my Shaman, allowed me to build up a muscle memory with my fingers and pay more attention to my surroundings. No longer did I have to focus on "when Proc A occurs do spell B then C unless proc X occurs then go to Z then A then B". No, for a DK the rotation is clear, and once in the groove I found myself, even as melee, looking around everywhere, trying to catch a loose mob with a death grip and pull it back to the Pali's consecration or interrupt spell casters with the choice of two different spell interrupts.
A smart person at the keyboard/mouse of a DK is a powerful and useful tool, and I found that the DK satisfied both my OCD (where a rotation was concerned) and my problem solving mind (where DK utility is concerned). For someone who really sucked at PvP I used my DK to help me break into the PvP world and feel more comfortable. I came to discover that DK's are more than just mashing 4 buttons over and over, owning a DK was an opportunity to be useful in ways that I hadn't before because I was not as aware of what was going on around me. My DK taught me how to be raid aware while in melee, which has translated to other melee classes I play with. With it's static rotation a player simply has more brain power to devote to everything else around him/her.
Being a greater part of the failure or success of a group has been extremely satisfying, and I owe it all to my DK. DK's are fun, if you have OCD; I do, so I'm having fun with mine.