New Things you want to consider with ODST
Of course, Bungie is using the same game engine used for Halo 3, but it was amped up a bit for few new features suchs as...
1. More Lighting affect. Reflective surfaces for Objects is increased along with increased shadowing effects.
2. A more realistic "Rag Doll" affect when an AI's die rather than the "Dropping like a cement block" feature.
Others things....
1. A more open environment to the player, choosing the objectives in any order, and fighting as the Elite ODST "Helljumpers" (Orbital Drop Shock Troopers) rather than a Spartan-II super soldier.

2. New variations of weapons such as the Silenced/Laser-sight scopped M7-Caseless Sub-machine gun (SMG), and the M6C/SOCOM firearm. The magnum is the variation of the M6C sidearm as seen in Halo 2, but it you want a detailed scrypt of the weapon, heres an excerpt from the Halopedia article...

3. Additional features for ODST's that Spartans don't have, such as the "VISR" mode (Visual Surveillance, Reconnaisance). A feature where everything in your surroundings is outlines to identify Friend from foe. for more additional information, heres what the Halo wiki has to offer...
The Visual Intelligence System, Reconnaissance, abbreviated to VISR, is an integrated data management system incorporated into the helmets of Orbital Drop Shock Troopers, able to perform a number of functions. It can link into UEG, CAA and UNSC infrastructure systems at the local, national, and global levels, administered on a need/rating basis, allowing the user to store and review up to 610MB of audio files, as well as street navigational data, recieve real-time mission data, and other important functions. It also includes low-light vision enhancement systems (ODST BIOS A29.817941B.01), raising the brightness of the surrounding area on the users Heads-Up-Display and highlighting personnel or equipment of interest - "friendly" combatants are highlighted in green, "enemy" combatants are highlighted in red, usable equipment and weaponry are highlighted in blue, while other points of interest are highlighted in yellow.
4. Also... you don't have shields for your ballistic body armor, but more of a stamina that needs to be replenish after you take hits... sucks for the people who rely to much on that.
Gameplay Add ons...
1. A new game mode called "Firefight" will be implemented in game. Its where you and your friends can play online or offline (system link works aswell) and fight off wave after wave of Covenant infantry in new maps similar to those in the games campaign. Along with this feature, armor permutations for ODST only (yeah!), emblems, Theater, and Forge will be coming back.
2. The 3 other maps of the Halo 3 Mythic Map Pack will be included (Citadel, Longshore, and Heretic) along with the original Halo 3 maps, Herioc, Legendary, the first edition of the Mythic maps, and Cold Storage.
More to come very soon!
P.S. I'm discontinuing the "What if Armor permutations had Gameplay affects?" thing. Oh, and if your curious about the video below, it was a parody of various of users (including me) of Halopedia.