One of the best features is that if your civilization is, well, civilized enough, enemy cities will actually just up and decide to join your empire, even if you haven't discovered where they are yet.
Also I think, but am not sure, that the same thing will also happen if they're afraid of you.
It's a bit too easy on the lower difficulty settings because of the way you can make "armies" out of units, and those armies gain "experience" and eventually "level up" to become ridiculously overpowered. The last game I played to completion, I destroyed everything in my path with a single army (unit) of Knights that had something like 30 victories, which causes them to get around +200% attack, another +100 against cities, +50 for having a "general" with them (a non-controllable "unit" that appears with the army when they've won enough battles), +1 movement, +1 movement after attack, healing after attacking, etc etc etc.