My theory is that BioWare used Earth as a symbol of how badly the Reapers have hit humanity... I personally don't think and I hope that the game will not be solely about Earth, because part of Mass Effect is about exploring different worlds

And it makes no sense that all the Reapers would attack Earth, I think the whole of The Milky Way is being attacked and they just used Earth, maybe to get the attention of non-Mass Effect players... They'll probably say "Sweet, I get to defend Earth from giant machines, I'm buying that game" :L
I want to see all the species unite and fight off the Reapers, It would be sad if ME3 was all about Humanity and Earth, I reckon that a more "Revealing" Trailer will be given at a later date
And I hope none of you have forgotten about The Collectors, Harbinger is not dead you know :L, with his "Releasing Control" right at the end