No matter what new strategy war game you play, you'll always come back to this one
AoK's X-factor can also be attributed to the game being the right length. Its successor, Age of Mythology, was ambitious in that it tried to make games a realistic half hour. However, AoM's rushed pace makes it stressful. AoM is superior in other ways though: it has a cleaner, more responsive interface and is better looking.
AoK got it right in every other way though -- AoK is the wheel; don't attempt to reinvent it, except perhaps a 3D graphic upgrade for modern times ala Age of Mythology.
I haven't tried AoE3 yet, because it's too American for me -- I am more interested in Feudal Times: swords, arrows, towers and walls. AoE4 perhaps?