Call me a lore noob (I've been called worse), but for some reason I never connected Arthas' charge into Silvermoon with everything going down in the Ghostlands. I've played through the Blood Elf starter area a few times now, and while it's obvious the Scourge is there, I never actually realized it was Arthas who brought them through there. Interesting.
And of course the story ends with Arthas "sealing his union" with the Lich King. We're still not sure (and maybe Blizzard isn't either) what exactly happened there-- did the Lich King completely possess Arthas, or did Arthas consume the Lich King? Are they still separate, and is there a chance we might still break Arthas free of the Lich King's influence? Blizzard has bolded, enlarged, and highlighted the "Now, we are one" quote, so it seems that a redemption for Arthas is unlikely, but Metzen hinted that we might be able to do exactly that at BlizzCon.
At any rate, it seems we won't know the end of the story until we actually step up to the Frozen Throne ourselves.