WTF is this game about anyway? Halo was already a FPS so what makes this so different from the last three games they put out?
This one, you play as an ODST. You don't have regenerating health like Master Chief. Once your shields are down, you have to find cover so they can recharge before your health takes too much of a hit.
Since you aren't a Spartan, you can't dual wield, but you do get a couple weapons that aren't in Halo 3. One is a silenced SMG and a silenced pistol, which is similar to the pistol in Halo: CE.
You have the VISR, which outlines things of interest...enemies in red, good guys in green, and important things in yellow. It also brings up a map in a bird's eye view, something Halo 3 didn't have.
There will also be Firefight, which akin to Gears of War 2's Horde and CoDWaW's Nazi Zombies.
It also will include a beta for Halo: Reach, which is a prequel to the entire Halo series thus far and will be releasing next year.
*edit*
Also, it originally started out as an expansion, that would be 3 to 4 hours long, but now it's supposed to be in the 7 to 8 hour range. As far as when it takes place in the entire storyline arc, it's just before Halo 3 takes place. The ODSTs are dropped into New Mombasa.
