This is a problem with all RPG ever. Even paper and pen one. Once you hit some level, it feels kinda idiotic to fight skeleton, kobold, wild dogs and so on. That's why Game masters (or Blizzard in this case) have to: set lowly enemy at our power level, risulting in some strange feeling (i killed a fel-guard alone on Argus but i can die from 10 murloc in new expac); or give us even bigger threath every time (also this can feel a bit too much forced).
Sadly it's something you cannot avoid. Specially now in WoW. You cuold only "reset" this with a time-lapse: for example, we deal with Sargeras, we skip 1000 years later and we are a new Hero, maybe the son of the son of the son of our native character.
To be honest, i don't see a lot of problem in WoW, because i simply don't think that we, as players, are Gods, we are simply one of the most skilled rapresentative of our class. But still, we only defeat big threat with: 1) Powerful Weapon; 2) Powerful Object; 3) Powerful allies; 4) And, least but not last, we fight with all other most skilled players from all classes. We raid with the best Priest, best Warrior, best Monk etc etc...
Only with this combined forces we are able to do something.
Meanwhile, in paper and pen D&D, you have insane OP combo that can utterly destroy everything, specially Game Master sanity XD