Group invites will be treated very poorly because the Care Package is empty.
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I agree. The gameplay felt so intense in CE, I was at the edge of my seat at every moment. The flood scared the hell out of me, but the introduction to the flood always made me come back.
I would take a combination of:
Halo 1's awesome gameplay, which kept you at the edge of the seat. I should never want to quit shooting enemies, and going back and repeating these encounters again and again just because the encounter was just so epic.
Halo 3's epic story, which had so much detail, vigour, and drama. I want to be drawn so much into the story, that I have to play the missions over and over to just listen to it.
Halo 2's... nothing.
[Edited on 11.13.2009 7:44 PM PST]