maybe the best game I have ever played to date!
The game starts pretty much after the first and (SPOILER) the Normandy gets attacked by a giant spaceship - you put all of the crew into the escape-pods, but you get thrown out of the Normandy and die - you wake up two years after and start working for Cerberus trying to find out what it is that keeps attacking human colonies. to do so you must yet again form a team to take down the enemies and save not just humanity, but the whole galaxy.
that is all of the story you will get out of me, all I can say other than that is that it is AWESOME and everyone with an Xbox 360 or a decent PC should play this game and experience the story.
there are some huge changes in the game and I will try to tell you about them all:
graphics:
the graphics are much better in this one and the framerate doesn't drop as much as it did in the first one, the characters move much more when you talk and looks more realistic and present.
Gameplay:
I said in my review of the first that it wasn't that good a shooting-game - that is no longer the case, now you have to press (A) to go to cover (as in gears of war) and it feels much better - the overheating has been replaced by reloading and ammo which is one of the points that I personally wouldn't have changed, but it is just fine.
You can now use your powers in real-time with the (Y), (rb) and (lb) buttons which is really a great new innovation to the series.
the Mako has been removed, but a downloadable vehicle is in the game for a few side-missions, it is called Hammerhead and to me it is kinda cool.
the annoying weapon and weapon-upgrade system from the first has gotten a complete overhaul, now you find some few weapons on missions and then you upgrade them on your ship with resource's from mining planets.
Mining planets is one of the more boring things in the game, but it is just okay - you have to put probes on planets to get Platinum, Palladium and two other recourse's.
Sound:
The sound is as awesome as it was in the first game - maybe a little bit better :D
all-in-all this is just a near-perfect game and one of the best games ever - I am almost 100% sure that this game will get my vote for GOTY 2010 :D