Good post. I shared many of the same thoughts in the official suggestions thread (excerpt copied below for anyone who cares to read). One thing I'd like to add is that the ending, as it stands, totally eliminates the desire to replay any bit of the series (and presumably any forthcoming DLC other than what will impact the ending).
1. ME3 is absolutely incredible. Epic is a term that is overused; this game truly deserves the label. I was a bit late to the party by the time I purchased my copy and therefore had heard grumblings about the conclusion of the trilogy before I started playing. I found the game so enthralling that I believed it impossible to have any sense of disappointment upon completing it. I was mistaken. The conclusion is a HUGE disappointment.
2. The disappointment stems from the fact that - unless I'm totally misinterpreting the ending - there is no point to the the struggle, the persistence, the triumph or glory of Commander Shepard. By the time Harbinger landed in London I felt as weary as the character I was directing. So much had been overcome up to that point and I was so immersed in it that I was not prepared for such an anti-cathartic stream of events throughout the subsequent several minutes.
3. To add further gravity to point #2 above, this lack of purpose makes it unappealing to play this or any other ME game ever again. As a general rule, I suspect people - though mostly speaking for myself - are greatly opposed to devoting 120+ hours to a cause only to fail. And again, based upon my interpretation of the ending, Shepard did fail. Some remnant of civilization may live on to some degree on an unknown planet but Earth was conquered. Humanity did not persevere, and it's because Shepard was never even presented with the opportunity to make one last stand; to deliver everything that was left of himself - which was clearly little by the final act - for the sake of all life. Fate was determined by a wholly unknown intelligence and the presentation of three essentially congruent choices. The expectation for a narrative spanning three chapters and unfolding over several years is not that it will fizzle in a moment of illusory choice. A hero like Shepard deserved a hero's death.
4. The nature of the ending, in and of itself, is not necessarily poor. I respect the vision. It is, however, entirely out of place in the context of the ME trilogy. Principally, what makes ME so appealing is the focus on character development, narrative and choice. In this sense, the trilogy is perfectly paced. Chapter 1 establishes the setting and introduces some of the primary characters. Chapter 2 develops the characters, adds a little depth and breadth to the main theme and muddies the waters between good/evil and right/wrong. The atmosphere in ME3 is a masterful composite. Choices from the first two chapters seamlessly create the story throughout the third and ensure that the story is one worth experiencing. And it is certainly that. It is such a story that every key plot point in ME3 presents as much intrigue and accomplishment as the finale of ME2. For this reason the ending of ME3 just doesn't fit with the rest of the journey. It is a low point in a series of high points. It is a moment of confusion and resignation amidst a story founded upon hope and resolve.
5. Harbinger was essentially a meaningless entity in ME3. Granted, it could be argued that the build up from ME2 was realized when Harbinger (possibly) killed Shepard as he attempted to reach the Citadel. Some form of conflict between the two would have added a little more purpose to this story arc.
6. Take back Earth? I will say what I suspect many others feel. The Mass Effect series contained more than enough despair for an entertainment medium that is typically reserved for enjoyment/escapism. As much as I hate to put myself into the "wants a cliche, predictable ending" crowd, I would have been pretty damn happy with killing every single reaper and liberating Earth. In fact, that's really the only ending that would have left me fully satisfied. The setting and the characters are so well done in the ME universe that it just seems a pointless waste to kill everything off (mostly).
All in all, the ME trilogy deserves every bit of accolade it's received. To say that it is a milestone of gaming is a severe understatement. It's just too bad it couldn't have concluded in a manner that endears players to the series. I'm afraid I will never have the desire to play through these games again.