Mortal strike, IMO, is definitely overpowered. It's not just the straight damage it can do, but that it cuts healing in -half- for 10 seconds. Warriors just do an insane amount of DPS, while still being able to maintain more armor than anyone else.
Add to that the huge increases you get from an item like Ashkandi, Sulfuras, or any GM weapons and you've got yourself an overpowered class. Yes, most warriors won't get those weapons, or have access to them anytime soon.. but that doesn't mean a damn thing. It just means that a few will be overpowered and the rest will be wondering what the big deal is, since they can't do it themselves.
If casting classes got that kind of damage increase from a single item the melee would cry.