A single player level created with UDK in 2 weeks time at VFS that features a cinematic dialog system complete with voice overs. The dialog system was created entirely with Kismet scripting and UI scenes.
My goal for the level was 'Every action has a reaction.' Enemies will shout out when they've spotted you, or when you drop out of their sight. Shooting a car will cause it to catch fire and explode. Running across the dance floor will cause it to light up.
The biggest issue with the level was the characters - due to time constraints, I wasn't able to create custom character models and so it's possible for the player to confuse who's who in a cinematic.