In the end, it's just a bit...boring...really
The basic idea of the game is to develop your settlement, churn out units and destroy the other team. Which is one of the main reasons why this game can get so repetitive and dull.
After you churn out your 10 or so riflemen, you send them marching off to fight a group of exactly the same types of units, then, a battle ensues in which each group forms lines and fires at each other. After this you are left with hardly any units remaining standing, so, you must churn out more units that will fall to exactly the same fate. There is little skill involved in these battles and there is little you can do to defeat the other except build more, and more units. This on top of the fairly poor graphics, with the zoomed out view not allowing much detail, makes battles very dull indeed. Of course there are the token upgrades, but these make very little aesthetic difference, and in the end it's still the same, very, very similar looking, riflemen, cavalry and mêlée units fighting and killing each other. There's just not that certain something about the military aspects of Age of Empires that can distinguish it from any other RTS game, and in an overcrowded market that the RTS market is, this is a certain aspect that this game desperately needs.
One part of the game that is impressive are the naval units of AoE III, the sight of your Galleons gliding between the islands of the Caribbean maps is something to behold, sails whipping and cannons thundering. Yet even this positive has a negative side. This being that whenever a ship nears the shore, it has a tendency to manoeuvre in very odd ways, for instance the sight of a Man-Of-War turning on a sixpence is very unrealistic indeed.
So in the end, the graphics, gameplay and nature of this game give little that will make you want to suddenly return and play it again, months after you thought you'd put the CD back in it's case for the last time. And that's the true sign of a very good strategy game, a game that is enthralling, entertaining and innovative, all things that this game is not.