Much as I adore Halo: Combat Evolved and can't seem to play any of my other games, because Halo 3 Multiplayer has such variety and depth, I doubt whether I will buy Halo: ODST. The weaker Halo 2 is being revisited. You are in a City (good), but you are being "told where to go" (bad). I would have much preferred a non-linear open-city with an active battle system (forces fighting each other across the streets in the manner of an RTS, some of it pre-scripted, other parts adapting to your presence... in the manner of GTA IV), but no, it looks like we are denied true innovation. Oh, and the helmet the Shock Trooper's wear looks stupid.
EDIT. I now have to retract much of what I just wrote the other day as I have now seen this video:
http://gamevideos.1up.com/video/id/25150/bigger
Which demos the open-city game, which has an "arena-style" multiplayer mode called firefight where you and your Xbox LIVE (or system link) friends battle an unending, semi-randomized, wave of Covenant dropships (Apparitions, I believe they are called). This appears to be in 3rd person, although the main campaign is in 1st person... although this may just be the way the action was captured (perhaps via a cinematic Theatre mode built-in to Halo: ODST). They comment in the video that it is like Gears of War Horde mode. Hopefully, it won't be 3rd person when you actually play it.
I'm still not overly keen on it. Your ODST doesn't have shields, but must pick up health packs. The visor makes everything look weird, and a bit like Metroid Prime. Given your relative weakness against Brutes, I don't really see how you can kill them so easily when they weren't a push-over for a Spartan like Master Chief. It is too late to change things now, but I would like them to consider downplaying the use of melee attacks, as I haven't seen evidence of Equipment the (X) button could do Melee, thus freeing up the Right Bumper to be 'Hold to Sprint' in the manner that is characteristic of Frontlines: Fuel of War - a game in which the gun is carried across the chest and cannot be fired whilst the character makes a limited sprint before fatigue gets the better of them.
This would help balance the game as I feel you would very likely walk into situations only to find yourself heavily outnumbered and too far from protective cover (at normal Halo movement pace). Sprinting would address this and give the game a "lighter" feel to Halo 1-3. One can dream...