ODST has two new weapons coming with it, one pistol and one machine gun. They're both silenced weapons and they both seem stronger than the pistol and machine gun from Halo 3. The pistol, in fact, was taking out Brutes with just one head shot after their shields were depleted.
Another big change with ODST is its non-linear focus. Pressing the back button brings up a real time map of one of the largest environments Bungie has ever created for a Halo game. There are multiple paths all around the city and multiple points of interest for the player to explore. Players can set down waypoints on the map and an indicator on the ODST's compass will show him which direction to go to get to his target. The city's AI will also try to be helpful and light up signs pointing the player to where he needs to go as well as telling him that there's danger ahead.
One final thing to note is just how amazing ODST looks. It's hard to believe that this is actually the same engine from Halo 3. They've had plenty of time to mess around with the engine and it really shows in ODST.