Age of Empires 3 is one of the best RTS games I have ever had the privilege of playing.
Ensemble has added quite a few new things to the traditional formula in Age of Empires 3, but probably the greatest new feature is the home city screen, the home city screen basically takes you to a view of your home city and you start out with a certain amount of cards that can be used to order things such as settlers, troops, or resources to your colony during whichever mission your on, you have a maximum of twenty cards that you can use for certain orders, and you gain cards by completing missions (usually you only get one or two cards). The combat for Age of Empires 3 is the traditional Empires formula, but this time you get guns (which is flipping awesome!) seeing as you are in the time periods of the Revolutionary War and forward (You do, however, start out mainly only being able to use swords and bows [and the like] for the first campaign).
The story for Empires 3, as always, takes you through certain events in history, but adds a fictional twist to them, such as the fountain of youth in the first campaign. Another neat thing about Empires 3 is how all the main characters are connected through long-running family ties. The graphics for Age of Empires 3 are pretty great but I've seen better in strategy games, graphics, however, don't often affect my view on games.
And The War Chiefs expansion is really a welcome addition to Age of Empires 3, and parts of the game are very-much-so superior to Age of Empire 3, for example (in one campaign) you go through the battles of Morristown and Yorktown during the Revolutionary War, which is one the best parts of all of Age of Empires 3, I will say that I like the look of the buildings much better in Age of Empires 3 than in WarChiefs.
So Age of Empires 3 to me is one of the greatest RTS experiences you can have in a game, and I would seriously recommend it to any RTS fans who haven't played it yet, or really anyone just wanting to have a good time playing a great RTS.