There are many issues with me3, however i think an important one is: the game fails to get the player emotionally invested in the plot - most importantly, in the characters, and in the EARTH IN PERIL main plot point.
EARTH, (and also ALL ORGANIC LIFE and THE ENTIRE GALAXY) is simply too large a concept for us to be able to form an emotional attachment to.
If you were to think of why earth blowing up right now might suck, you'll think about yourself, your memories, your friends, family, and so on.
So, you need to make what is at stake more personal, more accessible.
Now, Bioware tries to put a human face on this through the KID. However, this is such a heavy handed and obvious attempt at emotional manipulation, that the player simply rejects it right away.
I'm sure you all groaned when u saw the kid at the beginning.
Then later on, we are never given any evocative imagery from the crisis on Earth. We only ever see a hologram of Anderson, who TELLS us that things are in a bad way. We never SEE the crisis. Even when you land on Earth for the final mission, it seems pretty bland and boring. Not exactly captivating in any way.
What might have worked would be say... newscasts from Earth, images of an abandoned child crying in the wreckage of a house, of rag tag bands uselessly attacking a reaper with sticks and stones, of people crawling on their bellies trying to escape husks, of people being horribly captured, processed, along with the sound of crying, wailing, fire, and so on.
Little bits like this would bring the conflict on Earth down to a personal level, would get the player to become more emotionally invested in the issue.