We've got not one, but two Mass Effect 3 trailers for you: one showing the Reapers arriving on Earth, the other showing all the face-punching and gun-shooting you'll do once they land.
1) In Mass Effect 1, Earth wasn't even a real variable. Humans weren't the "star" of the universe, and they were still in the process of learning that the galaxy didn't revolve around them (sorry, Udina).
In Mass Effect 2, the Collectors targeted Earth but for justifiable reasons (raw population numbers, as shown in the Mass Effect: Incursion comic, not because "Earth is special"). The scale was reduced from humanity as one species among many, to humanity as plot driver.
In Mass Effect 3, the emphasis seems to be on one planet of one species, and on recruiting ally races to fight a fleet of an enemy whose single specimen nearly annihilated the combined on-hand fleets of the galaxy in the first game. I'm not really sure I understand.
2) Presentation-wise, I'm still as pumped as I ever will be.
3) Thank GOD, Shepard can finally roll!