"Come up with your ending before you figure out your middle. Seriously. Endings are hard, get yours working up front." Mass Effect 3... 17 suck endings. That got to be a record somewhere.
Even further on this tangent -- Mass Effect's problems began right at the setup. It starts too big. You begin as a hero, chosen by Earth to be, in essence, the representative for all humanity. (Recall your choice of back stories each points to your character having a past more interesting than anything that happens in the actual story -- always a bad sign and indicative of lazy writing.) It's a terrible way to start a story, it's a cheap trick (raising the stakes by giving them really big important labels). It's a testament to the game's execution that it is as good as it is despite this quite awful setup.
Given the scale of the problem (reapers), starting at a Spectre scale is pretty logical. Else you'd have character progression that is too fast