For me the only real unforgivable offense related to the ending was that the Catalyst scene simply didn't fit with the emotional themes presented in the ending, or even the entire game, or hell, arguably the entire franchise. - The Catalyst scene in it's entirety is cold, unemotional and purely intellectual. So much so that it asks it's viewer to literally become aware of, contemplate, and ultimately decide the application of the Crucible's next move which is going to have Galaxy wide consequences all in a couple minutes. ME3 is a primarily emotional story, with themes such as hope, love, friendship, finding one's humanity and what truly defines being a person. These are all things I felt strongly throughout each step of the story, most of all in the ending, which was sad, hopeless and very dark even with the requirements for the "best" War Assets score having been fulfilled. I get to see Shep's home world, my world, presented as nothing short of a apocalyptic wasteland, with death everywhere and everyone virtually out of hope and out of time. I play through the stage and watch heartbreaking goodbyes between Shep and her friends, a gust twisting scene with Kaidan, whom she loves where both are practically in tears and desperate not to lose each other again, and think to myself "is this actually happening? I heard the ending was bad, but this is so sad and emotional and not bad at all". I then continue through the second part up to what appears to be a heroic, final charge to reach the Citadel and watch Shep save her boyfriend, her friends and the Galaxy to boot but am surprised when a massive beam of destruction rains down, killing almost everyone in the area and see another scene with Kaidan which manages to be even more gut-rending than the first, watch Shep again try to reach the portal, only for her to end up be near dead with her Hard Suit seared in her flesh and barely clinging to life only to suffer ANOTHER emotional snap kick to the dick when she finally enters the Citadel and finds it transformed into a hellish nightmare looking like it came out of Hellraiser or Event Horizon. I watch, as she reaches the control center and has a powerful, emotionally significant and relevant showdown with her shadow self Illusive Man and where she's forced to gun down her pseudo father, barely activate the Crucible and finally the scene with her and Anderson which was probably most emotionally powerful of the entire franchise, only to have my tear factory and emotional rollercoaster slammed to a utter, slamming halt by her being magically lifted through the ceiling into a weird room that looks like a teleportation hub in an MMO complete with weird Sci Fi UFO music that sounds nothing like the other audio from any other part of the ending. Then, just when I think things can't get any stranger, some annoying, persnickety, bratty ghost child appears and starts talking about machines vs organics, ultimate solutions for the galaxy, and being inexplicably changed forever by the presence of the Crucible whose author's names he can't remember and expecting Shep to help him figure it all not. And just like that all the emotion the entire game has built up, the tidal wave of sad the ending has built up, is utterly disrupted by a awkward, sterile and emotionally lobotomized conversation with essentially a filler character who seemingly exists only to "explain" The Reapers and some weird, out of story pick a door style ending related choice to me. Even Shepard, who not just a moment ago was sad over her lost mentor and probably nearing death from wounds obtained reaching the Citadel is now standing upright and speaking in a calm and unemotional way that's completely unlike every other instance of her talking in the entire ending. Stranger still, she doesn't even seem to have a problem digesting the mental barf of pointless revelations this crazy AI child is throwing her way, or the prospect that her ultimate goal she's been fighting for three installments of the franchise to realize apparently might not actually be the goal she should be fighting for, but instead might be the goal of her crazy, indoctrinated nemesis or some completely unheard of, misplaced sounding mishegoss about everything in the Galaxy becoming a Cyborg. Then, just when I wonder if I'm hallucinating or really did fall down the virtual rabbit hole it's all over, the creepy space brat and his price is right style final solution is never again mentioned at any point in the ending, and everything goes back to being sad, feeling important and plays out, basically, how I imagined it was going to after the the last moments with Anderson.
Obviously, I know everything isn't for everybody, and I stand by my previous comment that the ending wasn't THAT bad, especially not compared to most Sci Fi series endings. But the Catalyst conversation was the most weird, pointless, irrelevant, dross bunch of nothing I think I've ever seen, and if nothing else is the ultimate example of developers needing to trust in the fact they made an emotional story that mattered to it's fans, made characters who you could be invested in and root for, and that their story about a talented but mostly ordinary woman who never lost her hope no matter how bad things got, fought like hell with the people she loved and ended up somehow beating the odds one last time and saving life as they knew it was enough. That it mattered enough, that it didn't need fake attempts to be deep and philosophical, that it didn't need contrived, cliché mary chosen one implications with Shep that never previously existed anywhere and actually sort of defeat the point of what makes her awesome, that it didn't to convince me to somehow care more than I already did, because the truth is I already cared more than I cared in any other game/tv show/movie I've seen to date.
Sometimes it's best to trust in what you've made to speak and succeed for itself, and sometimes simple and sweet is actually better. Leave the pointless information, silly gimmicks and pretentious revelations to other inferior series who tryhard like that because they have to, and let the ending be about stopping the reapers, what became of Shep, the characters everyone came to care about so much, and the Galaxy they all worked so hard to save, like it deserves. I really wish there was a way to just hack everybody's computer and install MEHEM or JAM as a mandatory prerequisite for playing ME3 so the conversations about the ending can be about it's other flaws, or strengths, or anything, whatever.
TLDR; I think the ending was decent, but the Catalyst scene in it's eternity is pointless and completely out of emotion and off topic with the rest of the ending and ultimately the rest of the story, and that JAM or MEHEM should be a required install before playing ME3.