I'm really more on the fence about this game than my score indicates. It's really more of a 3.5 than a 4.
As for the Pros, first of all it's gorgeous. The game utilizes different lighting very effectively which provides different regions of the American hemisphere their own unique aura (for the most part). The Yukon winter, the subtropical fauna, the Caribbean waters, and the New England fall are all portrayed very well. It's also nice to see such a variety of regions portrayed (who else would have thought to include Patagonia but Ensemble?).
Next, the graphics engine really is a pioneer among RTS games. To see a cannonball shot from a culverin, scream over the landscape, smash into a town center, blow out the windows and a corner from the building's steeple, come out the other side, and come to a rolling stop a few relative feet from the building with splinters falling around it sets a standard that no other RTS from here on out can afford to not meet. No more scripted death scenes no matter the style of death. In AOE3, if you get plugged by cannonshot, as in real life you don't fall dead where you're struck. You careen 10 yards back, flop around, and then you die. It opens the door for more realistic (and potentially horrific) gameplay than any other RTS has.
Also, I like the combat style. Some don't because it can take away the hold-your-breath moment when two armies collide in hand-to-hand combat like AOE2 was able to preserve. However, AOE3 has done a good job at making the stand-around style of 17th, 18th century combat more exciting than it could be otherwise, particularly with visuals supplied by the smoke registering from the musket barrels. Unlike other games such as Empire Earth where soldiers of this time frame appear to just flash lights at each other, in AOE3 there's no question that the two sides are shooting weapons primed with gunpowder. This is especially true on the high seas with ships firing broadsides at one another.
However, I shall also use this point to start highlighting the Cons of the game. First of all, armies during this time period tended to be very ornate and march almost in parade fashion - even in the face of fire. Flags flew, drums tapped, officers shouted commands to their troops, and those troops, especially the regulars, tended to be disciplined, disciplined, disciplined. Even in the New World. The game should have provided these kind of units (and not just via the scenario editor where they currently exist). Flag-bearers or drummers could provide combat bonuses so as to encourage their presence and keep them from being superfluous. It's been done before (think Lord of the Rings).
The game should have also provided a larger variety of formations on both land and water. It has effectively taken a step back from AOE2:the Conquerors which opened up some formations to armies and finally allowed you to keep those capricious and moronic ships from going off on their own and getting in each other's way. Now we're back to the original AOE. Games are supposed to evolve - not devolve.
Also, and this is one of my biggest complaints, diplomacy is gone from single-player. Completely. In an age where international law was born and treaties played such an important part in activities in the New World, I find it incomprehensible that Ensemble, so committed to providing a balanced perspective on history, would decide to boil this period down to something a hair beyond cavemen clubbing each other. What they did in AOE2 was fine. You could start off allied, neutral, or an enemy and you had full control over that at any time. You could potentially bribe someone to be your ally (or at least not your enemy) or you could help them to defeat a common enemy and then turn around and stab them in the back. It kept you on your toes because, like people tend to do, nations could change their minds. Not so here. In skirmish mode, you're either someone's ally or their enemy. There is no neutrality. There is no bartering for favors. The game is simply black and white despite its many colors. AOE3 devolving again.
I'm also not fond of the campaign. I think the story-line campaign, with its semi-interesting plot, doesn't do near as much for the history of the New World as separate campaigns based on the lives of key figures do. Because of AOE2, people now know more about El Cid and Saladin than they would have known otherwise. AOE3 could have done campaigns based on the exploits of Pizarro or an Incan leader (to avoid a recap of the Montezuma campaign in The Conquerors), the Marquis de Montcalm, a whole campaign devoted to Simon Bolivar, Pontiac, or on particular wars such as Queen Anne's War or the Seven Year's War (and not just a shoddy blip in the campaign story that it currently is). I know that the game isn't supposed to be just a pure history lesson but the previous AOEs did nothing wrong with those campaigns. In AOE3, Ensemble has attempted to fix something that isn't broken. This point of view is further vindicated by the return to the AOE2 campaign format in The Asian Dynasties.
To wrap it up, I love the choice in time period, I love the graphics, and I like the combat (I don't buy the arguments that the fighting ends up becoming just a mesh of humanity when things get hand-to-hand. That's what ALWAYS happens. Even for those armies who like to stay in formation and keep discipline e.g. Roman legions, hand-to-hand always becomes messy and disordered.). But this game is like a ditzy cheerleader - nice to look at but not much substance.