Okay lets put this in perspective shall we?
The reapers are used to arriving at the citadel and then expanding out to try and gather as much genetic material as possible whilst shutting down all of the mass relays to stop people getting actual help. This plan (the usual one as Ilos does not seem to have occured at any point prior during the cycles of exinction) has now been fantastically derailed and they are now going to be unable to ensure that the newer races are unable to communicate with each other.
So they send through Harbinger to do something in secret that they probably would have done overtly once they had taken over the citadel "assess for genetic opportunities". They focus on humanity and find that Whaddayaknow! These hairless apes will do just fine in making a reaper! Harbinger assesses the other races and basically goes "meh" to them, and so kills Shep and tries to get the corpse.
This fails, shep comes back and kills the collectors. The initial plan now completly gone to hell in a hambasket the reapers decide "you know what, fuck it" and decide to use brute force to win back the galaxy from these pesky "new races". So they decide to complete their usual plan but on a more local scale, especially in regard to Humanity. So they go after earth to take out the Systems Alliance high command and to take over the other human colonies, because then this disaster will not be a total waste.
They send out other ships to other areas ofc, but its earth they focus on because not only is it the most populous human planet, it is also the one which (if they can cripple it) will make the others easier to subdue. Now its up to Shep and his/her band of misfits, psychopaths and assorted arsekicking individuals to stop their plans on earth and then save the rest of the galaxy.
I personally really enjoyed both final battles, shooting games always have difficulties in making good final bosses and Mass Effect is no exception, but the big fight up the citadel and against Saren was a hell of a lot of fun and the big fight up to the Reaper Fetus in 2 was interesting and tense even if the final boss was easier to beat than a sackfull of kittens.
And as a final thing Sean Bean voices the guy in this trailer... AWESOME!