The newly re-named Halo 3 ODST doesn't exist. It looks good on the drawing board, but that's all it is so far - ideas, concepts, and a very early game. But it's unlike anything the Halo universe has seen.
ODST is only a few months into development. Nobody has played or seen anything of the game beyond a few glimpses of concept art and the original teaser. With only eleven months until release, you'd normally be right to tremble at just how underdeveloped it seems, but this is a special case. It's short, you see. Very short. Clocking in at "between three and five hours", more likely the former.
Still, though short, the three hours are divided between linear Halo-style levels and a wide-open hub world. Playing as the unnamed Orbital Drop Shock Trooper just minutes after arriving in a ruined New Mombasa, you'll search for the rest of your squad - four team-mates scattered across the open city - using a PDA map and the ODST vision mode, highlighting foes, useful items and waypoints. Reach a waypoint where one of your squad fell and you'll play through a flashback mission: a straight, linear Halo campaign mission telling the story of how the trooper died, or escaped to leave only their weapon or helmet in the debris.
Each flashback clocks in at about thirty minutes with the rest of your time spent in the open world, evading foes and preparing ambushes. Though the unnamed Trooper gets a new vision mode, map, and silenced SMG, he lacks some of the tools Halo types are accustomed to; he's a regular guy, so that means no shield, no dual-wielding, no motion tracker, no huge jump, slower walking speed, and shorter stature.
Instead, in the open world, ODST is a game of smarts and strategy where a one-on-one fight with a Brute will end in squishy death. It's not Halo: Advanced Warfighter, but it's the thinking man's Halo, and sadly, Bungie's final visit to the Haloverse.