That phrase equal to some romantic **** in melodramas.
No doubts every class can be good in good hands.
But again, in situations where mage-rogue-feral are playing piano on the keyboard, DK players can faceroll with the same success.
That was confirmed by game designers, and, as result, DKs were nerfed in some next expansions, wich forced all these "every class is good" theorycrafters with main DK to reroll (there was an official statistiс regarding class smigration and overall situation).
Anyway, there is no point to discuss bouta Dk's faceroleness, it's obvious. Personally for me, Blizzard's opinion is more important that private server guys opinions, especially when they are biased.