In the latest Mass Effect blog update Casey Hudson, Project Director of BioWare talks about how difficult their situation was at E3, because they wanted to show a deeper look on the game without spoiling it:
"We've had a lot of great feedback from this year's E3 demo, but it was a long and treacherous road to get there. As E3 approached this year, we realized that we had a problem. We've talked about how "Mass Effect" puts you in control of emotionally-charged decisions that have a huge impact on the game world and the way you experience the story. But we realized that it was time to show a deeper look at what we meant by that. However, all of those situations in the game are, by definition, major spoilers. How would we show these amazing moments without giving away something that players would want to discover on their own?"