That you would use the word "amazing" to describe Mass Effect 3 (With out irony) is proof that you don't (or don't want to) understand how deeply or on how many levels Mass Effect 3 failed on. So I'm just going to list the failures in order of grievousness.
1) The complete lack of roll playing or player agency.
You could roll play very effectively in the first two Mass Effect games. I should know I played 7 different Shepards. Each with their own unique personality and morals.
When I jump into ME 3 all the dialog choices are boiled down to paragon and renegade options.
And for the most part your which ever choice you make is irrelevant because the Par and Ren option conveying exactly the same message.
Only having 2 dialog options means that you'll run into the awkward "But Shepard wouldn't say either of those things." moment a lot.
Also (and arguably less importantly) you'll hear all the combinations and permutations of dialog options after 4 playthroughs.
In the first 2 games Renegade Shepard could be a real prick. In ME 3 if they survived the suicide mission they're Shepard's BFF. It seriously threw me for a loop when my stone cold renegade Shep was just spilling her heart and soul to the unconscious Kaiden, without even consulting me with a dialog wheel.
2)Shift of focus from talking to listening.
There are about one third the amount of dialog wheels in ME 3 as there are in ME 2
You initiate all of your unimportant fetch quests by eavesdropping on someone's conversation. When you complete their quest you say something along the lines of "You can pick up your macguffan up in the cargo bay" The meaningless fetch guests in ME 2 were memorable and interesting. (The Krogan who wanted the Presidium fish, The Volus who lost his credit chit, The Asaris who wanted fake ID's) Do you even remember the Volus who wanted the Book of Plenix? or the doctor who wanted the Rings of Alume? No? It's because you didn't talk to them.
This even happens aboard the Normandy you get 2 dialog sessions per character. And no that is not neutralized by listening to them spout off 1 liners in between missions.
If you look at any mission in ME 2 the combat is pretty evenly broken up with talkie parts. ME 2 even had non combat missions. In ME 3 you hardly get any talkies during missions.
3)Negating important player choices
Why is Udina a councilor? I remember giving Anderson that seat.
Why is Cerberus trying to kill me? I gave them a collector base.
Why is the Rachnai queen alive? I killed her on Noveria.
Nothing changes whether you saved the council or not
Nothing changes if you turned down Spectre status in ME 2
Nothing changes if you cheat on your love interest (Repeatedly)
It didn't matter if Thane's son got arrested
It didn't matter if Tali got exiled
So much you do was just pointless
The absolute lowest point in the game is when Cerberus assaults the Citadel (Yes lower than the ending) because it was so easily avoidable by choices that you've already made in previous games. If Anderson was a councilor Udina wouldn't have been able to orchestrate a coup that easily. And if Cerberus was on my good side they wouldn't have attacked the Citadel. And it goes back to the complete lack of roll playing at the end of the mission. Maybe you want a human to take over the council. (You already saved the council's life once and it did bugger all to endear them to you, why give them a second chance.) Maybe Udina seizing power isn't such a bad thing. But they don't give you that option.
That Citadel mission is also where you meet Kai Leng and he kills Thane because Shepard stands there watching them fight (Presumably thinking "So that's how Assassins say 'hi'")
4)The ending Nope. ME 3 would still suck even if the ending was good.
4)Cheep enemies, shit controls.
almost every attack in Cerberus's arsenal is remarkably cheep. Landing grenades in your lap from half way across the field, setting up turrets that shoot you to shreds with infinite shields. Smoke that some how obstructs Biotic and Tech powers. Phantoms that flip around like freaks so much that you can't target them and instakill you the second they're close.
The Geth are hardly better with invisible shotgun Hunters and Primes that throw turrets behind cover.
And the cheapness of the enemies is just exacerbated by the fact that pushing "A" on the controller will do any one of 4 things with complete and utter randomness. You'll either start sprinting, take the nearest cover, do a barrel roll, or vault over cover if you're already in it. People usually just say I'm retarded or something but I've never once been able to do what I wanted Shepard to do when the A button was involved. Every single one of my deaths (Not an exaggeration) can be attributed the A button making Shepard do the wrong thing.
5) Tali's picture
Yeah, I know it's petty but it would have been less offensive if Tali took off her helmet and we just saw
6) The Crucible
I know I said I wasn't going to talk about the end but the Crucible isn't part of the end now is it. It's introduced in the beginning of the second mission and building it and moving it is the entire point of everything you're doing in the game. Even though nobody knows what it does. Not even you. not even after you beat the game. Not one person in any previous cycle knew what it did or how it worked but they built it anyway. And added to it's design? No one was ever able to finish it, so no one ever used it, so no one ever knew what it was supposed to do, but they still added to it's design. Does that make any sense? The Crucible is the plot hole singularity.
Some time before the Prothean cycle it was made to incorporate the Citadel, which the Reapers built, and is run by the Keepers (Who, until the Protheans were under Reaper control) So if someone before the Protheans tried hooking this thing up to the Citadel the Keepers would have told the Reapers about it.
So the plot holes are bad enough as they stand, but even perfectly executed the Crucible still introduces a jarring tonal shift into the series. Over the course of one line of dialog during the Mars mission the tone of the series changes from "Desperately fighting an overwhelming force with slim to no chance of victory" to "Build the magic 'win' button and push it"
7) The over use of Cerberus
You fight them about twice as much as you fight Reapers and about four times as much as you should (That calculation is disregarding the fact that you shouldn't be fighting them at all if you gave them the collector base)
Some examples. Cerberus shows at an STG base on the Salarian home world for no reason. Cerberus knows about an ancient Turian bomb that the Shadow Broker didn't even know about. In ME 2 it is made abundantly clear that Cerberus has limited resources and they almost broke the bank bringing Shepard back from the Dead. They should be struggling financially but they appear to have more money and power then they did last game. At one point in the game James says that Cerberus spies were working for the Collectors during the events of Mass Effect 2. (That line alone negates every thing ME 2 is based on)
8)Jessica Chobot has no business voice acting
That one's kind of self explanatory
9) They made planet scanning suck.
I didn't think planet scanning could get any worse. I was wrong. Oh god was I wrong.
9A)They took out Hacking and bypassing
I don't even know why they would do that? What did bypass ever do to them? Hell they insisted on putting Kinnect functionality into their game, why not make Kinnect hacking and bypass that might have been fun.
9B) there's no planet exploring
At least ME 2 had the Hammerhead Tank to drive around some places, but ME 3 always has you on foot.
10)I'm absolutely sure I forgot something important.
ME 3 has so much wrong with it that I can't even keep all the problems strait in my head.