It's most likely the familiarity of the universe that makes playing Halo 3: ODST such an initially jarring experience. Because despite the familiarity - I know if I take out that Brute first the grunts will just run away - it's a very different game.
Chronologically ODST is set during the events of Halo 2 and Halo 3, and if you're a follower of the overarching story you'll no doubt appreciate the nods and references to the events of the other games. One of these comes as you drop to the surface of Earth and find your pod sent spiralling off course in the wake of a Covenant ship entering Slipspace. Unsurprisingly this leaves you a little worse for wear, and explains the state of New Mombasa when you awake.
Having been out cold for a few hours, the aptly nicknamed newest addition to his squad, the Rookie (known only by that moniker throughout), has become somewhat separated from his team mates, who he needs to find. This means wandering the lonely streets of New Mombasa, which is overrun with Covenant forces. There's an overwhelming dark and oppressive feel to the city which imbues a sense that all is not well.
As a result, while I'd have been all for taking a run and gun approach with Master Chief under my control, this time around I was inclined to avoid conflict wherever possible. Thankfully there are plenty of overturned vehicles and, yes, chest high walls to cower behind when you're left with no choice but to fight.
Another reason to adjust your combat style is introduced early on in the game. If you drop from some height to the ground you'll start huffing and puffing and a bar at the top of your screen (or visor, if you prefer) will highlight the finite nature of your health.
Unlike Master Chief, who could take a massive pounding with nary a complaint, the Rookie has a small amount of shield to burn through before further hits will eat into his health. At this point you'll be compelled to find a health pack for two reasons. First, low heath is bad - obviously. Second, and for me more pressing, you'll be subjected to an annoying beep continuously until you heal yourself. After a couple of minutes this get's very, very frustrating, which is probably intentional.