Yeah it is pretty dumb. Going through the prologue with your father would help significantly.
I haven't watched Star Wars outside of Phantom Menace so I have no idea what you're talking about. Well I watched spoilers so you see the Kett being plenty menacing. As for the Archon you see him try to imitate Alec and stalk off angry when it doesn't work. As for the Reapers we have three games of them. Not to mention they're plenty idiotic too ("It is not a thing you can comprehend!" except it is. And it's dumb).
Again poor writing is a fair complaint. But BW games have plenty of poor writing so acting like this is some revelation that ruins everything else baffles me. Playing ME to begin with means you have some tolerance for poor writing.
A New Hope is the most typical hero's journey out there. Luke's a regular guy, you see him as a regular guy, and then the big bad kills his parents, establishing them as a threat. He goes through some character development and then the mentor figure (i.e. ME:A's dad) is killed off and he's left to solve things for himself. ME:A tries to cut down that entire 2 and a half hour movie into a short prologue mission and
still fails to do it. By the time I became Pathfinder I: didn't care about Sara's dad's sacrifice; knew the Kett were just another enemy, one I could beat; and didn't feel at all like Sara was out of her depth because in gameplay I'm still Shepherd.
Your point about the Reapers doesn't make any difference because the stupidness of the Reapers only came in with ME3. By the time you get to the Citadel in ME1 the Reapers were a clear and obvious threat; in ME:A the main threat of the prologue is lightning.
The previous games had poor writing, yes, but they had stories I wanted to actually continue with. Bioware has tried to do too many things with ME:A and, going from the prologue alone, haven't done a good job at accomplishing any of them. Poor writing is one thing, not being able to accomplish the basic Hero's Journey is another. I didn't particularly care about the generic "big bad vs. chosen one" story shown in the trailers but I could have looked over it if the writing was good, it isn't.
EDIT: To clarify further, that random kid's death in ME3, for example, was a terrible plot device and a poor motivation for Shepherd but the Reapers were already a threat, they didn't need introducing. ME:A is an attempt to reboot the game's universe, it can't afford to fail in making us care for the story.