So let me start off by saying that I'm not a member of the internet rage brigade on this one. At the time of playing it, it felt like an acceptable ending. Not good or by any means great, but okay. I may have benefited from lowered expectations in this respect, since from the level of anger I was expecting something End of Eva-like -- that is, an ending that was incomprehensible on a literal level. This was obviously not the case.
I'm also not here to do technical nitpicks or lore-related inconsistencies. First, because there's already a million people doing that, and second, because honestly it's not that important. If the ending had worked on an emotional/dramatic/game design level, the audience would be willing to give the game the benefit of the doubt, and questions like "Doesn't blowing up the relays destroy the associated systems?" would be in the same category as "Doesn't blowing up the Death Star rain billions of tons of debris down on Endor?" Star Wars is a good comparison, actually. When it's good (Empire Strikes Back) even gaping plot holes are not a problem. (The infamous "How long did Luke spend training with Yoda?" question.) It's only when it doesn't work dramatically (the prequels) that people start with the technical stuff.
(I'm not convinced that anyone really cares THAT much about technical stuff. If they'd included a line like, "Don't worry, I'll activate a safety system on the relays so they don't blow up stars!", would that really have made things better and satisfied the internet rage? It's trivial to handwave almost any technical complaint with technobabble.)
What I would like to talk about is that emotional/dramatic/game design level. Obviously this failed to work for a great many people, so there must be something going wrong, and speaking from an author's perspective it would be nice to know what. So, is there a particular place where things fall apart?
Yes, it turns out -- right when Shepard gets hit by the Reaper beam. Let's follow along.
So, you fight your way through devastated London, confront the Reaper destroyer, and fight off a few waves of Reaper troops. This is important from a game design point of view since these fights effectively serve as the final boss. (Aside from the slow-motion Marauder...) Personally I would tend to argue that this doesn't make for a terribly satisfying conclusion: fighting five or six Brutes is fair enough, but the wave of Banshees you don't even fight, just evade until the Reaper comes close enough to shoot with missiles. It's not a bad encounter per se (although the crappy cover/roll mechanic makes it harder than it ought to be) but as the final battle of the game a "run and hide from the giant beam gun" lacks a certain something.
You hit the destroyer with missiles, it falls over, and then you head towards the giant transport beam. Everybody sprints toward it, various tanks and soldiers get disintigrated, etc. All fine so far. The theme of the whole ending section is something like "willing sacrifice" -- the image of those guys running forward while their buddies get vaporized all around them is actually quite a powerful one. What we expect, at this point, is that Shepard & Co will be the only ones who make it, and therefore as usual have to save the day by themselves.
Then, Shepard gets hit by the beam! Expectation subverted -- a neat trick, if you can pull it off.
Unfortunately, it's too soon. If the ending were structured so that reaching the beam was literally the last ditch -- getting there would be the final victory -- then this would work fine. Shepard nearly makes it, gets hit, barely survives, and staggers in to victory and death. But that's not where we are in the ending. This is only the first step, we assume, and there's a bunch more stuff to accomplish after that. Once Shepard wakes up all bloody and slow-motion, though, she (mine was a she) is robbed of what a writer would call her 'agency', and that's when everything falls apart.
All right, so you make it to the beam, and you're up on the Citadel. There's some needlessly confusing dialogue here with Anderson, which has been interpreted as support for an Inception-style "it's all in her head!" theory. To me it just smacks of either lazy or last-minute writing -- they wanted to get Anderson into that scene, but didn't want him hanging out with you as you walked along, so they sort of awkwardly shoe-horned that bit in. At this point the ending flow is a bit off. Shepard is hurt and barely able to lift her gun, so obviously any super boss-fights are out, but most of the important issues aren't resolved. Again, the problem is that Shepard is crippled too soon, reducing the player to a totally passive role as the most important events in the series play out.
You meet up with Anderson and Martin Sheen/TIM, who demonstrates his power to take control of people using Reaper tech, I think? Again, this is a really awkward scene. It doesn't say "conspiracy theory" to me so much as it does "ass pull" -- it works on a literal level, sort of, if you assume TIM now has the power to control people like puppets, but that's obviously not something we get any notion of earlier. (Personally it seems like a missed opportunity to have TIM become a Saren-style Reaper construct and have a final boss fight, which would have a pleasing circularity to it.) You shoot TIM, or waste him with your diplomacy.
The problem here is that this scene doesn't work, dramatically. It plays out like Shepard is somehow in control of things, and TIM is trying to convince her, but that's manifestly not the case -- there's no reason he couldn't just shoot her and get on with things himself, for example. This is the problem with the whole ending sequence: Shepard is given the choices to make, because she's the protagonist and that's how the game works, but there doesn't seem to be a reason for it other than that she is the protagonist. Crippling her took away her agency, and now they're awkwardly giving it back, but without a good explanation as to why. You're not imposing your choice on TIM, he just lets you win, for some reason. The "argue him into shooting himself" actually kind of makes more sense there, but if the goal was "TIM has Shepard at his mercy, but she convinces him of his error," then it's really badly handled.
Anyway. You sit down, Anderson dies, Shepard is taken on a beam of light to meet the god-like AI that lives in the Citadel. (A note for the technical nitpickers -- much has been said about how the layout, etc, of the machines in this section makes no sense. If you want to assume things are happening in Shepard's mind, THIS is the time to do it, since presumably it would be well within the AI's powers to pop Shepard into a virtual environment so they could have a chat. The three branching paths strongly suggest the layout is allegorical. But anyway.) First off, having the AI assume the form and in particular the voice of a child is a very bad idea, since while their kid voice-actor gives it his best shot, he's not really up to it. I get that they didn't want to have just another booming machine-voice, but still. If you're picking dead characters out of Shepard's memories, you've got plenty to choose from. Why not Kaidan/Ashley?
The kid gives you a very confusing explanation of what's going on. Again, this feels nothing to me so much as "rushed". It would have been a great place, for example, for the kid to explain a little bit about his own history, the creation of the Reapers, etc. Give us another beacon-style memory dump, something like that. We lack vital context. This is important, because Shepard is then asked to decide the fate of the galaxy based on nothing more than what the kid just said. I can't stress enough how problematic this is. Dramatic choices are created when the choosing character is fully aware of both the stakes and the consequences. For example: a million action movies to the contrary, the red-wire/blue-wire bomb-defusal choice is not actually that dramatic. We know that what happens is ultimatly in the hands of the writers -- they can do anything they want, since the character/audience isn't fully informed. To create a really dramatic or tense choice, you need to inform the character (and thus the audience) of what will happen, or at least what might happen, so that we follow what's going on in the character's mind.
Here, we have extremely poor information, delivered by a possibly-untrustworthy source. It's the equivalent of the final choice being "Door #1, Door #2, or Door #3" -- because we haven't been prepared beforehand, we're too aware that our fate is completely in the hands of the writers. The illusion of control is broken.
Further, once again we have this awkwardly shoe-horned agency. We're never told why the kid allows Shepard to make this monumental decision, other than being "the first organic to get here". She gets to pick because she's the main character, full stop, and as a result the scene doesn't make any sense. You can't construct a plausible motive for the kid to offer her the choice (at least without inventing a lot of background information that isn't provided) and so we have to fall back on "whim of a god-like alien". Again, this is okay on a literal level -- aliens are weird, fine -- but dramatically it doesn't work.
You make your choice, and get your red/green/blue ending. Some people were raging about the destruction of the relays, but that has seemed inevitable to me from the end of ME1 -- the relays were Reaper constructions to begin with, artificially guiding the development of civilization, so it makes sense thematically that they would have to be destroyed to escape that cycle. However, it's pretty obvious that your choice involves enormous sacrifice on the part of, well, everyone in the galaxy, and so what's missing from the ending is some sense that it's worth it. This is where the kid's missing context comes in again. His dialogue implies that organic/synthetic conflict is inevitable and hugely destructive, and I'm prepared to believe that, if you show it to me. Or even tell me about it.
I mean, speaking as a sci-fi buff, it's easy to envision a scenario where any synthetic life eventually evolves out-of-control and achieves god-like status, threatening all organic life in the galaxy. It's essentially a Singularity problem: weakly superhuman AI will sooner or later become strongly superhuman AI, so as friendly as the Geth seem now it only takes one of them going exponential to screw things up. So, that would be fine with me, if that was the story they told us. But, instead, we get nothing.
If that context was present, the final choice would have some actual meaning. Will you destroy the Reapers and end the cycle, taking a chance that the kid is wrong? Do you admit the kid is correct and take control of the Reapers to run things yourself? Or do you take the third, synthesis option of evolving to a new and possibly better form of being? The elements of a good conclusion are there, but the missing pieces are enough context to understand the decision, and some reason that Shepard is being offered the choice in the first place.
The final cutscene is also needlessly confusing. Nice to know that Joker and EDI and all escape, fine. (Although -- in my run, Liara was with me on Earth, then you see her leaving the ship with Joker ...) But at least a nod in the direction of the rest of the galaxy would probably be a good idea. Show us the various worlds with people marveling at their newfound synthetic-ness, or the Reapers exploding in the skies of Thessia, or whatever. We need some indication of what happened in the larger context, or it's just confused. Probably this is also a case of ran-out-of-time/money.
One thing that is definitely required is Shepard's death. People bitch and moan about that, too, but it's been led up to the entire series. The whole theme of Mass Effect is "sacrifice" and hard choices, and Shepard's self-sacrifice provides the final capstone. Still, again, a little more context as to why this is going to happen would be nice, or possibly at least one "bad ending" choice where you can save yourself with catastrophic consequences. Without that, the emotional impact is lessened.
So, what's the final conclusion? I think there's nothing wrong with the ending per se, on a literal level. But emotionally, what should have been an enormously impactful sequence is compromised from the start by some bad design choices and weirdly awkward writing. It does leave me speculating about budgets and deadlines, but that's neither here nor there. It will be interesting to see what they do with the new-ending DLC, at least...