What most impressed me (the lighting was a close second!) was the event. I'm not entirely sure I know exactly what it was, but to me it looks like, if a bunch of people are playing alone or with their friends while connected to xbox live, and they are near the same area, there's a chance for the event to start. Then all players are connected into one game with what seemed like lossless transistion, and a huge boss spawns somewhere. Everyone near the area swarms in to this one spot, creating a constant reinforcements effect. If they actually manage to pull it off while having the transition lossless, it would be truly amazing.