Core gameplay remains pretty much the same as Rome, with better graphics and slight improvements to AI.
The greatest improvement is obviously the graphics, provided your computer can handle it. Units are much more detailed and varied and the scale of things is more impressive. Cities look like real cities now, complete with houses, churches, workshops, etc., smoke rising from chimneys and so on. A huge city will really look huge, absolutely sprawling. Each city also tends to appear unique to some extent, with a specific layout and architectural style.
The units look more varied than those found in Rome, with variations in helmets, shields and armor (usually 3 kinds of each, with additional variations in color), so no more clone wars. Normal and specular mapping were put to good use and bring more realism to each unit, cloth given the appearance of texture and metal armor made shiny. The animations are a big step forward as well. Units will be seen blocking attacks with their shields or sticking a sword through someone's gut while they lay squirming on the ground. I was impressed at how well-coordinated the animations were. You'll never see a unit swinging at an empty space... he'll always be engaged with another unit in a believable fashion. The environmental effects are quite nice.
Different times of day look as they should - sunsets create a reddish glow over everything, etc. It rains, snows, may be foggy, or you may be in the middle of a sandstorm in the desert.
The AI has been improved in some ways, but unfortunately there are times when it breaks and rather badly. Oftentimes, it breaks when encountering a large number of missile units. The enemy will tend to charge, stop, retreat, re-charge, stop, over and over until they're all mowed down. If you move melee units forward to engage them, they break that cycle, but otherwise, they go nowhere. Also, when the enemy AI is overmatched it tends to retreat to high ground and just sit there, allowing the player to surround its army. There are also some problems with sieges, as the AI tends to run right up to the walls and get its units picked off by the missile defenses. The entire opposing army will rush forward into missile range as soon as you move one unit outside the city/castle, which creates a bit of a bait-and-shoot exploit. As soon as they get close, you move that unit back inside, the enemy army slowly turns around and walks back to their positions, getting shot in the back as they go.
Despite these flaws, I still found the AI to be a considerable step forward from Rome. The AI can now make decent attempts at city sieges. They will build siege equipment, ladders, rams, towers, and will use them fairly effectively. I've actually lost a few cities to the AI (albeit outnumbered by at least 3:1 in those cases), which is something I can't say for Rome. The AI also does a much better job defending cities. I no longer see units haplessly running up and down the streets dying under a rain of arrows. The walls are usually loaded with missile units and spearmen behind them to counter the ladders. If you're sieging a city and it's going to fall next turn, the AI will come out and attack you as a last-ditch effort. Cavalry is smarter - they'll charge, pull back, and charge again, which I don't remember seeing in Rome. There seem to be fewer instances of single-stack armies running around, but it still happens.
Even though there's nothing groundbreaking here, it's still worth checking out just for the spectacle.