I have never been able to make sense of death knight mechanics. I can recite them: abilities consume runes, which produce runic power, which is necessary for other abilities. In practice, though, when I play a death knight it always eventually boils down to button mashing, which is not fun at all. So, despite having rolled a death knight at least three or four times, each in an attempt to finally stick with the class, I've given up. Death knights and me, we just weren't meant to be.
I still don't understand why, though, because honestly the concept behind both death knight and rogue mechanics is really similar! It doesn't make any sense that I'd be perfectly at home with one class but not the other. What gives, brain? I feed you and give you sleep when you need it, the least you could do is be more consistent in your virtual-world data processing, okay?! Alas, I cannot change the natural order of things, and death knight mastery will likely remain my World of Warcraft white whale. What about you, fair readers? What is the hardest class for you to play?