You know, it's awfully suspicious that Kil'jaeden's plans with the orcs concluded just when Sargeras' plans required them to be available and left them in a condition most suitable for Sargeras' purposes. It's also suspicious that the Legion's plans for killing and resurrecting Kel'Thuzad as a Lich left the Sunwell corrupted, un-warded and in perfect condition for Kil'jaeden to use it as a portal several years later.
I can't tell if it's excellent writing by Blizzard to showcase the Xanatos like efficiency of the Legion or if they just got lucky.