They compliment each other, as you attack some missions Rambo style but approach Mombasa with a more reserved, explorative attitude. You can take on the ten missions in whichever order you want, and the game rewards explorers with sound files that detail an entirely separate story of a distressed young woman - a drama you can uncover as you scour the streets.
This new structure is the game's key difference. Bungie has heavily touted the differences/weaknesses in being an ODST instead of a Spartan. Does it make a huge difference to the way you play the game? No, is the simple answer.
You can't jump as high, but it's barely noticeable. Explosions and melee strikes knock you back a few feet but that's no biggie. Your melee attacks don't do as much damage, which often means that Brutes will survive the elbow to the face that would previously have laid them out in Halo 3. That's not a game-changing difference either.
You no-longer have a shield, which would have been an epic change if Bungie didn't just substitute it with 'stamina', which basically does exactly the same thing.
When you're shot the screen goes red and when it's at its deepest red, further shots chip away at your health bar. Hide away safe and your stamina returns - so it's like having a shield and a health bar, just like Halo 1. Fall damage is back too, also like the original Halo.
What else has changed? The SMG from Halo 2 is back, only now it's silenced and more accurate. The pistol in ODST is like the one from Halo 1 - powerful, accurate and very useful with its scope, but not as unbalanced as it was in the original.
There seems to have been some other subtle changes to the existing weapon balance - shotguns don't feel as powerful at point-blank range, but conversely the assault rifle seems more useful at mid-range than it was in Halo 3.
The ODST's visor sight is cool. When activated it highlights enemies, friends and significant objects with different coloured outlines, which act as a visual aid particularly in the dark streets of Mombasa.
Otherwise though, ODST has mostly the same weapons and enemies, and because of that there's an instant familiarity to the way it's played. But that's not a bad thing. This is a genre-defining series, remember? Your Halo-loving brain will be happy.
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There aren't any bottlenecks like the insane third mission in Halo 3, Tsavo Highway, which was like trying to head butt your way through a brick wall on Legendary. The flashback missions throw you into one magnificent set piece after another, often against greater numbers of enemies than you've ever seen in Halo.
The Covenant AI is still the best and most lifelike of any FPS - fighting them never gets boring or repetitive. You can see them making judgement calls, working together and using the scenery effectively. And the finely-tuned weapon system that goes with that is the other half of what makes Halo such a pleasure to play - a finely balanced system of 'scissors, paper, stone', where different combinations of weapons are needed to deal with each situation strategically.
So good are Halo's AI and weapons that there's a whole new multiplayer mode built on that basis. Firefight mode is absolutely fantastic. The idea is simple - you and three other players fight increasingly difficult waves of Covenant in one of ten maps - but the result is utter class.
The twist is in a rule-modifying skull system - with each passing round the game activates different game-changing skulls that, for example, make the enemies throw more grenades, or change the health system so that you have to melee a Covenant to recharge your stamina. It's brutal.
Your team shares a single pool of lives, so one rogue can ruin it for the whole squad, making it important that you look after each other as drop ships spit out dozens of enemies all around the map. We played it in LAN with a bunch of busy, quiet journos and still loved it. Playing online with mates will be awesome.