Where to begin indeed... and huge part of Halo is co-op and the marines. However useless they may be later on, they provide comic relief, a sense of companionship on a remote planet, and a psuedo-reason for you to haul ass and maybe save one or two with the reward of one being a sniper. Along those lines, co-op is just amazing and I don't believe this game would have done quite so well without it. Games are always better with a friend, and sometimes playing with someone can be better than playing against someone. The sheer scale of the game is also incredible. Not only the massive environments, but also within the story. Halfway through the game, you get used to how the Covenant think and react, and combat starts to get a little dull. Welcome Flood, who require completely different tactics to beat, and are the type of enemy you really hate. Everyone laughs at grunts, and kinds of shrugs off most of the elites, but no one likes the Flood especially with the massive chain reactions or death they cause. There's a lot more that makes Halo what it is like the fluid controls, great physics system and variety of fighting you do, but I'm too tired to go into that now. ;)