I would say the lan partys I have with my friends because I always own on them all night long. Also to see the graphics during game play, just to see everything put together and perfect. Also to play the new maps and try the new guns.
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Hmm...Yeah...Everybody pretty much hit the nail on the head. XD I'll just say story development and online. I love Halo already, let's try playing and getting my ass kicked online! :P