so how would you all have liked it to end ?
For a start I would never have moved the Citadel, or had the Citadel as the catalyst. Having the Citadel suddenly move to earth was far to conveniant and created a plot hole you could drive a tank through. At new point did anybody go "omg all those people on the citadel" when they would obviously have been wiped out by the Reapers. Infact it it were me I wouldnt have invented the idea of a Catalyst at all, an all in one I-WIN button smacks of a lack of vision and creativeness in the writing.
For me I'd have kept the assault on earth, the galaxy unites, a final showdown against all the odds with the mainstay of the Reaper fleet, lead of course by Harbinger. First off start with an epic space battle, Reapers wreaking havoc amongst the allied fleet, the battle is all going wrong, they're just too powerful.
In a balls out move Shepard slams the Normandy face first into Harbinger, the Normandy is ****** but remains barely intact. Shepard and the crew disembark, lets try and blow it up from the inside out. Wave after wave of little Reapers rally to try and repel the intruders, all the while you're getting updates from the battle outside. The turian line is broken, they're trying to save what little of their fleet they can... an Asari cruiser is going down, captain slams it into one of the other Reapers, etc etc.
Your party battles deeper into Harbinger, you can hear him speaking to you, taunting you, you're weak like all the other organics etc. As you go along you destroy various things that look vital, basically shooting anything that you can. You arrive at a central brain hub, you're facing an avatar of Harbinger (much like Saren was an avatar of Sovereign) you battle, its long, several stages, you hear more reports of battle outside.
You defeat the avatar of Harbinger, you confront him, conversation options about why the Reapers are doing this. Its all explained (I dont know a valid reason for this, it has to be better than what they have given us so far) - you're given an option to save earth, humans have potential, we wont complete our cull of earth if you're willing to leave Harbinger alive and abandon the rest of the galaxy to its fate. Your other option is to destroy Harbinger from the inside out, commiting humans to a battle against the Reapers with the rest of the galaxy, a battle you most likely cannot win. More conversation options, can you change the Reapers mind?
I dunno, something along those lines.