I'm going to begin by commending Martin O'Donnell on what is possibly his greatest score to date. He has managed to maintain the distinct Halo feel, while creating something completely new and unique. The score is perfectly matched to the different tone in this game; a bit more somber and less sweeping than previous Halo scores, it evokes images of old murder mystery movies and serials. However, when the action ramps up, so too does the intensity of the music, perfectly matching what is happening at all times.
The score is not the only audio elements Bungie has gotten right in ODST; the sound effects create a texture new to the game, alien weapons sound more alien, creeping around at night sounds...like creeping around at night. The voice acting, though not fantastic, serves the story and doesn't distract you too greatly from immersing yourself completely into the environment. One complaint I have is when Buck is injured, the heavy breathing is too loud and doesn't seem to sound right.
Halo 3: ODST is much slower than the earlier entries into the franchise; it is very possible to spend some serious time not at all engaged in any action, especially at night. This is of course the point behind the game, you are no longer the cybernetic super-soldier and must avoid heavy action accordingly. One of the new tools you are given to accomplish this is a new visor mode that will outline enemies, friendlies, and objectives in high contrast colouring. I found the visor to be absolutely essential and great fun at night, and almost useless during the day.
During the daytime segments you switch roles to other members of your squad and are prone to some more intense battles. Because you aren't the walking tank anymore; these fights can be quite tough, but not in the same crazy fun way of other Halo games. Your characters are too weak to be facing the same overwhelming odds with a few marine backups that the Master Chief could shrug off with ease. Often instead of being sneaky and working my way through the groups as I could in the nighttime, I was forced into a run and gun scenario and found myself wishing I was someone else, or at least had a working visor.
The theatre mode is the same as in Halo 3, giving you the ability to rewatch any of your matches or games, and control the camera while doing so. ODST is still unable to rewind or clip single player games, but I assume you can with the firefight matches. Single player videos tend to be far less exciting than those in Halo 3, mainly due to your need to sneak around and lack of awesome stunts you could pull off in the older game. I don't foresee much use for the theatre in single player, however from the sounds of firefight there will be some amazing video clips coming out of the multiplayer soon.
Visually ODST looks much the same as Halo 3, being built on the same engine I assume. The menus retain much of the same layout as those in Halo 3, with some slight cosmetic changes to reflect the military intelligence and detective nature of ODST. The loading screens are made up to look like your map, or the SUPERVISOR's screens, but I found myself wanting the old halo screens after my second mission as these new ones look too much like your start menu for my taste. Characters in cinematics for ODST are much more awkward than those in Halo 3, making many unnatural moves and poses.
As I mentioned at the beginning of this post; these are all based on a short play period with a small portion of the game and I'm sure when I revisit it later I'll have much more to say on many of these subjects. I haven't even had a chance yet to sit and play any Firefight, something I am dying to do because it looks so cool. At this point, I cannot recommend Halo 3: ODST for any newcomer to the Halo franchise, I feel it would be far too inaccessible and the problems would far out way the pros. Of course, any fans of previous Halo games should already have ODST and be playing it right now.