-The Normandy crashes onto a strange planet, over half of the crew dies. You're stuck with Jacob and Miranda.
-Mass Effect's intro turns into a "horde" mode where you and your party fight until everyone is dead.
-Once everyone is dead, you get a new party. The story of this new party isn't revealed at all (wait for prequels), and you're their newest recruit. They're generic, helmeted humans who are space marines. Everyone is male, and everyone has a bland, monotone voice. They're also all very patriotic and show it. They're not on this mission for Earth, they're on it for the USA.
-This new party goes to investigate the Normandy crash site and find everyone dead. So they fly back to Earth to fight the Reapers (they weren't so important to keep on Earth).
-The space marines find an entrance zone at the belly of a Reaper ship, and when it opens to let other ships out, it goes in. Now you're just in a corridor shooter, fighting all kinds of husks, until you reach a boss fight and you fight another gigantic Reaper-terminator-look-alike.
-You save the world, but there's a cliffhanger: Your party's ship's power goes out, and everything is dead within the ship after you fly out of the Reaper's belly. You're floating out into space, and no one notices. Then, you all go into hibernation chambers. "To be continued..." flashes on a black screen and some screamo band plays the credit music.
Mass Effect 3 ruined enough here?
Oh, and you only get two different guns, there's no customization of any kind, no dialog option, and no third person view.