I played an unholy DK through Outlands and northrend the first month wotlk came out.
You started in outlands with gear would make angels weep. You got 4 or 5 diseases (I cant remember, it was alot) that could be spread among as many mobs as were near with a button. You had an awesome pet that only required a corpse to summon, and in the unlikely event that you died you came back as a ghoul, just to make sure you could exact that last little but of revenge. BGs in the 40-50 range were basically decided by how many DKs you had on your side, and i distinctly remember participating in a contest to see how many of those buzzards we could grab, at once, outside honor hold and still finish the battle with full health (it was 16).

Notice all this stuff is in the past tense. Today, are still extremely powerful classes but only on paper. They have a litany of (albeit watered down) abilities that can do some serious damage when mixed the right way. However, they have also become, arguably, the least creative class Blizz ever made. Whereas the stealth mechanic rogues get requires a certain subversity (dare i say, perversity) to fully harness, the mage an understanding of CC, the warlock health and inventory management, etc., DKs have become basically about stacking debuffs to launch a single massive strike. When I see a DK, I know that the fight will evolve in only one way: diseases, chains, melee strike. There is no subtlety, no creativity, in the way a DK fights. Every time I go to EoTS, and watch the shamans eject the enemy off the middle with TB or Priests MC their foes off bridges to their deaths I feel a little pride, because I have seen an extremely powerful but limited ability employed in just the right way- a certain alchemy that is sublime to behold.

So therefore I say do not hate the DK because he has leet DPS, weep for his pitiable position- his godliness has reduced him to an existence of banal ownage, never understanding the joy, the exult, of using a skill in a way it was never intended but instead merely meting out his damage in a most unbalanced way.