The trailers and marketing for the Halo games are brilliant because Bungie is great at making certain moments in the game. But fitting in those moments to the over all story and plot progression, not so great. There are a few melancholy moments between the Chief and Cortana (amazing music, great directing, good dialogue and some great voice acting), but Bungie didn't know how to make these moments worth anything in the end. There's no pay off. And I blame that on Bungie being over cautious with their last game. In trying not turn off any fans, they played it too safe by not doing anything remotely outlandish, exciting or surprising. What you have left is a game exactly how you described: barren. Halo C.E and Halo 2 were definitely pushing the cogs well and fast for Halo 3's plot to be ambitious. I mean, after pushing Gravemind the way they did, his demise is so forgettable. In fact his entire role in Halo 3 is forgettable. He's the LOTR film's Sauron to the Halo trilogy.