Dark Knight and Death knight can not be ripped off because they are a part of everything. It's like the science fiction world now; everything has been done- you're going to see repeats.
Neither FF or WoW should be allowed to claim first rights to the job, even though FF came out with Dark knight before WoW came out with Death Knights.
Hell, one can even say that WoWs Murlocks which look astonishingly similar to the sahagins used in FF1 (way before wow bish). FF does not have the right to claim first rights however, for myth wins that battle.
However, the blatent WoW ripoffs of chocobos from FF must not go unpunished! Mechanical or not!