***** edit *****
Ok i'm getting tired of being asked the same questions so I'll try to lay out my theories for all of it.
- Starvation issue. The Reapers target large cities, not rural areas. The
whole 'OMG Quarians and Turians will starve' thing has already been
shown to be overblown as D-proteins occur naturally and can be
synthesized NOW so you KNOW they can be made with better tech. Will some people have it tough? Sure, but they'll survive which is the point.
- Relays blowing up. The
idea that the relays blow up the solar system has already been
addressed and is refuted by the different destroy endings. Obviously the
crucible CAN fry planets (BAD destroy ending) but if you built it well enough it doesn't(every other ending). Simply put, there's a difference between releasing RAW energy ( slamming an asteroid into Alpha ) and the
crucible converting the relays energy into a form of radiation. Basic chem teaches that the larger the radius you have to energize, the more diffuse the
energy is going to be.
Whatever the crucible releases is some form of
radiation that bathes the entire star cluster the relay is in which is
1000x of times larger in radius than blowing up a single solar system. It's converting that raw energy from the relay and turning it into a controlled burst not unlike a radio station tower.
- Citadel falling to earth. Lastly
the whole, nuclear winter from the citadel falling back to earth is
wrong too b/c the citadel has little to no kinetic energy behind it. We
fear extinction level events from asteroids because they travel 25km/s
or 90,000km/h. It's the kinetic energy behind the asteroid that makes it
the threat, not the asteroid itself.
From what I witnessed in trailers, rural areas were attacked by the Reapers. It's all speculative however, a safer assumption would be every major metropolitan city and the surrounding vicinity were most likely razed.
By far my biggest concern has been the availability of resources. ME is set some 200 years in the future, and what with the population skyrocketing due to the apparrent advances in medical tech (medi-gel?), resource acquisition and allocation has got to be a sticky subject for the Alliance. I assume (again speculative) that this problem was remedied with the discovery of the Mass Relays, which allowed humans to colonize other garden worlds and thus ship much needed resources back to our solar system for use. Very fine and dandy until the destruction of the relays, which cuts us off from those needed resources.
Now, with the combined fleets of the United Galactic Anti-Reaper Squad trapped in our system, the available pool of resources just got a whole lot thinner. According to my playthrough, there should have been small groups of Hanar, Drell, Volus, Elcor, Salarians, and Vorcha present when the Citadel fell. At the same time, substantial populations of Asari, Turian, Krogan, and Quarians were also present.
Putting aside the Turians and Quarians for the moment, you now have eight other races competing with humans for sustenance alone on a war-ravaged Earth. Now, while the human population is most diminished (actual numbers absent), it still leaves a rather sizeable potential for conflict. And yes, there will be conflict. Conflict is the one constant of social interaction. It is inevitable. There will always be one person/being with a differing opinion. Always. That is not speculative, it's cold, hard truth. And now while the newly enjoined races may get along for awhile, it is inevitable that sometime down the road, one of these races will come into conflict with another over the limited amount of resources available for use. And this is also assuming that infrastructure recovers to the point where resources can be properly allocated for usage among the diverse peoples. In the face of the reaper invasion, it would most likely take a decade in order to: organize leadership-assess and pool remaining resources-institute mechanisms for harvesting/mobilizing resources- (perceived) fair distribution of resources.
I could see the races getting along peachy during those first phases, riding on the eclectic rush of such an epic victory as each race selects their finest representatives to help rebuild earth. But towards the tail end of assessing what is useable, what do you have? Coal? Fields needing to be resown? Livestock that needs tending? Most of the resources needed to put this in motion would have been located in the metropolitan cities, which unfortunately are now in no condition to contribute anything meaningful to the rebuilding effort. You could get the people working the fields, but they'd need to be fed, but the food would only last you so long, and agriculture takes time, which with limited (and depleting) resources, time would not be in abundance.
The foodstuffs and advantages we see today are only possible because of an intricate network of infrastructure that exists on a macro level. The american farmer depends on fuel to power the machines that work the fields with water pumped in from pipes which grow the crops, which are then sent by more machines to cities all over the globe to be consumed by people which build the parts for more machines that use the fuel provided by other people etc. etc. Now, take out a few of those 'nodes' With no machines, the farmer cannot farm as efficiently, slowing or reducing production. If the machines controlling the reservoir which pumps the water no longer function because they are destroyed? Then the farmer is at the complete mercy of the weather, again slowing or reducing production. My point here is that the Reapers have essentially blasted earth back to the era just before the industrial revolution. You said in an earlier post that you could fashion a wrench out of wood. Yes, but how long would that wooden wrench last without industrial-grade reinforcement? And what happens when you have exhausted all the proper trees that could have made suitable wrenches because you have eight different races all burning through wrenches to try to make repairs on ships/new homes/new factories etc.
In terms of resource usage, what would be left to process what we do have? Saw mills in the PacNW? It's a fair bet that the industrial centers of production were destroyed by the reapers (major cities right?) Even if you could organize the myriad of races to cooperate on the levels needed to properly mine/grow/cut the materiel needed to rebuild earth, there's no feasable way of processing said materiel into useful product. If the Reapers were smart about their genocidal attack (which they obviously are, given they've been doing this for quite some time), they would've taken out every major damn, nuclear plant, coal plant, anything that provides power to humans. (By the way, I know I saw computers and crap still on in London when I went back, but who's to say there aren't small backup generators keeping things online, which are in no way going to be powerful enough to provide the energy needed for a full scale planetary rebuilding effort). And with that, just how are we supposed to feed the Quarians and Turians if we can't properly extract/process the proper foodstuffs for them? Unless food replicator tech from star trek exists, the Quarians and Turians are going to have an awful time while they wait for the proper tech to come along to synthesize d-protein foods from a world largely populated by life evolving along an l-protein scheme.
I look at the state of resources today and just try to imagine what it would be like if a) WW3 broke out, only worse, leaving every major city in the world as a smoldering husk (pun intended),

if the infastructure that currently keeps this world viable would to suddenly cease. and c) if you were to tell everybody at once that it was their duty to help rebuild, no ifs ands or buts. The result is not pretty. Not only would N. American and Asian farmlands have to feed the entirety of the world's populations, they would have to figure out a way to distribute said foodstuffs in a timely and fair manner to everybody on earth (yikes India!)
I haven't even touched on the effects of crime/social deviance upon the post-reaper world. And how about the mentally challenged/physically challenged/disabled? They require personnel and resources. Veterans of the war missing a limb or two? Do they get to skirt out on duties? How does the hungry Quarian feel when Sgt. Sanjay Arashpur, who gave his arms and legs fighting husks in London, gets to sit around all day while Seelai Toras vas Quib Quib has to collect his droppings every time he yells her name?
I could go on, but WALLOFTEXT OMG.
To sum up, I don't care about how the relays should've obliterated Sol, nor do I care about the destruction caused by the citadel ruins (actually I do, but my wallotext is out of control). To me, the mobilization of the diminished resources of our system, and the limited amount of proper facilities with which to process said resources, in the face of inevitable social tension, is what kills the ending for me.