Halo 3: ODST is trying very hard to do something different to the other Halo games – or at least the shooter-centric ones. It doesn't want you to feel like an unstoppable killing machine and at every turn it gives you a little reminder that you're not the hulking cyborg known as Master Chief.
Instead, ODST wants you to feel isolated, alone and confused. It wants you vulnerable and afraid. Achieving that is the game's first major success and something that singleplayer game seems to excel at imparting; constantly hammering home that you're stranded in a big, dark and unfriendly city and that you are just a man with a gun against an entire army of aliens.