I like Nibuca's little writeup recently from her blog asking what happens when we become just too powerful to care? Just like her, I've played a full, months-and-months, session of D&D before, and by the time your characters start to flirt with level 20 (the maximum level in that system), you're so powerful that the story barely makes sense any more -- you're crossing planes of existence, unweaving and re-weaving the fabric of reality, and taking down gods, more or less. Once you've vanquished evil from the earth four or five times, yet another threat doesn't bother you so much.
And to a certain extent, that's exactly what's happening with World of Warcraft -- when the game first started, the devs casually threw out there that it would take 40 level 80s to take Arthas down, which was of course a guess based on what raiding was at the time. But nowadays, we're all level 80, you only need five people to go after Arthas, and very soon, even someone like Deathwing will seem conquerable. In the next expansion, we already know that we're going to transverse some planes of existence, and when you're a being that can do that, why bother fighting frost wyrms? Just escape their reality and/or will them out of yours.