The mission we got to see was an assassination of a powerful lawyer who is holed up at his residence. The player had to find a way into the house, up to the top floor, and kill him, all without alerting a single guard. (The mission was not a total failure if alarms were tripped, but apparently it changes for the worse as the lawyer escapes and has to be chased on foot.) Here, though, Corvo used light and darkness to avoid the guards and took the lawyer by surprise in his chambers, finishing him off and making a daring escape off of the balcony. Guards and the city watchmen gave chase, but our assassin is a crafty type who used powerful abilities, a bit of impressive first-person melee combat, an ability to point at a nearby ledge or wall and instantly leap towards it, and a little old fashioned parkour-style clambering on rooftops and such to evade the guards.
What I liked seeing was the interesting combinations of powers and weapons that could be employed. Using the Stop Time power, Corvo could move around but his weapons' projectiles would freeze in space once they left the barrel of his flintlock-style pistol. But a few well-placed shots in this mode did wonders, as once time started back up again, several enemies would die at once from well-placed bullets that, for a second, seemed to be just floating still in mid-air.