I literally just want to know what the big deal was
If I remember correctly, the ending debacle was about more than Shepard's sacrifice - though that was a part of it for several players.
Because the relays were destroyed, people thought the other races were stranded at Earth (since fixing the relays, even if it was possible, would probably take forever) and the dextro races would probably starve since some of them had hundreds of years to travel to get home (without the relays). People were also upset with how easily Joker seemed to abandon Shepard as well as the thought that the crew was stranded on some backwater planet were some of the crew would starve. The end mission was also criticized because it was just a slew of fighting the Reaper's zombie-like creations +star child and differently coloured endings. Added to that no one knew what happened to the companions that ran towards Citadel-elevator-beam, did they die? Also, people thought that Harbinger was severely underutilized and disliked that he was suddenly so quiet after being such a chatty foe in ME2.
As for the Star child, people didn't like how he came out of nowhere, that he looked like the child they felt was a contrived attempt to force emotion out of the player, or that you couldn't question or argue with him (not much dialogue wheel options).
For the choices. Before the game come out, I believe a Bioware employee had stated there would be 16 endings so that was what people were expecting. Instead they got 3 endings that while they were different felt alike (which is were the cupcakes came from, cupcakes with the same flavour but differently coloured frosting -blue, green and red- were sent to Bioware to emphasis that though superficially different when it came down to it all the endings were the same). Also, the synthesis ending felt a bit like you were forcing the whole synthetic/organic thing on the galaxy (and what happened to the husks that regained their minds?) and the destroy ending forced you to destroy the Geth and EDI after you could have spent a whole lot of time and effort forging peace between the Geth and the Quarians.
Also, people argued, using the Arrival DLC as reference, that the destruction of the relays logically should have devastating consequences and since there was one relatively close to Earth everyone would have died from that fallout. Added to that, we didn't get much information about what happened after that so all we had were the fans' speculations that painted a pretty bleak picture for the galaxy we'd tried so hard to save.
Those are the main points I remember. Personally, I didn't manage to avoid the spoilers that Shepard was going to die - but I did manage to avoid all the other spoilers floating around due to the leak. I was actually looking forward to it since I like a good heroic sacrifice but the endings didn't really deliver back then. I had hoped I would get more information of the after, perhaps a heroes funeral, maybe some of the companions reminiscing, etc. At the time, the endings felt like a punch to the gut.
The extended cut added removed several of the concerns players had though and salvaged the endings for a lot of people. I still don't really like the endings but I've gotten over it and the reaction Bioware got really was too much.
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On topic:
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I agree that Bioware painted themselves into a corner with the ME3 endings, but at the time they clearly weren't expecting to make another game. I don't agree that it has ruined anything to move to Andromeda. People would have been irritated no matter what they chose to do, whether it was picking a canon ending, setting it in the past, setting it in the far future, or leaving the galaxy altogether.
As for the tempest, as someone pointed out, ships with the same function tend to look alike - The tempest could even be based on the Normandy which was an experimental prototype if I remember right.
Dragging the old races along is a positive thing for me, they're a huge part of what I loved about the old Mass Effect. The world of ME wouldn't feel the same without them.
As for the arcs looking like the citadel... Isn't it pretty reasonable? I mean the Arcs (and they're going to be several) are space ships where a lot of people are going to live. The Citadel is, granted, a space station rather than a ship but it's a place capable of holding a huge number of people. Since Andromeda is so far away and no one really knows what to expect, it's probably a good idea to have a base that's fully capable of being at least a temporary "home" until there's a chance to get settled - or even find worlds that are capable of being settled.
As for N7 characters, at first I felt it was a bit forced as well but if humanity really is desperate to find a new home it isn't unreasonable that they send along some of their elite soldiers -the best they have to offer- to help make sure it succeeds.
And working for an official organisation... well, that's just personal taste. I don't have problem with it.