I like dks. I love them. Now going to offer my opinion on some things i think need discussion for.
In wrath the meta was different back then for arena. Skill actually mattered. So unholy was better then frost.
Not saying frost doesn't take much skill, or takes too much skill, it's just with frost you don't have a lot of room for your skill to bring the spec up to a victory. So yea it's kind of hard to win with frost because it's lack of utility prevents you from using your skill to carry the spec to victory.
The warlock is just mad, because dks are effective against casters classes to a degree.
Also the meta for mist of pandaria has changed. In arena, the meta is fast paced, pop all coldowns and burst fast type of play style. There is no room for thinking, or using ultility when burst 1st and not die is the way of the meta.
Dks are a solid class because their resources are on an infinite, but short coldown mechanic, and they have the skills that really matter like dps, burst, cc, healing, defensive coldowns. Not roleplaying !@#$ like water walking, or utility stuff like healthstones or summoning portal.
Dks are strong combatants, just that the meta doesn't really favor any strategy.
Hope that helps.
Helps a lot. Guess people don't actually read and just see the title. Thanks for the help man. If people read they would know im not defending Frost. Just trying to get across that the high horse of thinking they are good because they don't play dk is a little funny.