I love how so many people here these forums care so little about Humanity, when they are bloody humans. Nothing but a bunch of sell outs, all of you. Earth is the human home world, damn right I'm going to fight for it.
It isn't that people object so much to there being a focus on retaking Earth. It is that there is no justification given for it in the lore, and no justification in ME3.
The Alliance is a fourth-rate power behind the Turian Hierarchy, Asari Republics, and Salarian Union according to the lore. Palaven, as the homeworld of the most militarily powerful species, and Thessia, as the most economically important planet, should have a strategic importance many times greater than that of Earth. So why is the counterattack occuring at Earth? And why should the Turians or Asari send their fleets to Earth when their own homeworlds, each of which is more strategically important than Earth, are under attack?
That isn't to say that having Earth be the focus of ME3 was inherently wrong. Just that if the writers were going to go that route, they shouldn't have ignored their own lore. You need to give a justfication for Earth being the focus within the plot, whether that is hitting a Reaper weak spot (more Reapers being deployed to Palaven and Thessia) or some other reason that makes Earth a better option for a counterattack at that time.
Having Shepard tell the Turian Councilor that he needed Turian fleets to save Earth at a time when Palaven was burning, with no further justfication beyond "because humans," was as silly and ridiculous as having Shepard try to convince Ashley to join a Cerberus ship at Horizon by sayng, "C'mon..it will be just like old times."
It also followed on the heels of other aspects of the series where the humans were presented as special, even if it required ignoring science. For example in Mass Effect 2 in order to make humans special snowflakes, its said that the Reapers are focusing on humanity because of their great genetic diversity. This was a "LOL Wut?" moment, as humanity isn't genetically diverse at all. In fact we are one of the least gentically diverse species on our own planet.
In conclusion, the less 'humans are special' ego-stroking in ME4 the better.