Beyond what we played, we got a demo of how you'll discover uncharted worlds in the game and what you can do with them. Once you have the Normandy at your disposal, you'll be able to go and explore anywhere you like. If you discover a planet, you'll typically get some kind of side quest that's either unique from the main story or, in the case of the demo we saw, one that is related to something in the main plot. The demo we saw found Shepard stumbling onto a pirate outpost and interacting with their leader before turning it into a pirate butt-kicking situation.
This is nothing new for a BioWare game. Making choices which change how the game plays out was a defining feature of previous titles Knights of the Old Republic and Jade Empire. But the choices in Mass Effect are much more complex and much harder to make, as studio president Greg Zeschuk explains.