I think they went wrong pretty much right away in their belief that Mass Effect 1 was about exploration. Mass Effect 1 was not about exploration, but instead it was about discovery and decision-making. Discovery means finding something new and going somewhere no one else has been before, or at the very least finding something that has been lost a long time. Andromeda's planets are all settled, and after you get through the first vault, they all pretty much feel the same. Exploration is a personal experience, you haven't been there before, but other people have recently or currently are there, discovery is more transcendent. It's easy to think that the two words mean the same thing, but there is a difference, and the failure to understand this is what makes Mass Effect: Andromeda lacking. While exploration can be interesting, we all have a greater desire to discover. Mass Effect 1 takes you to places like Ilos where no one has set foot in millennia, whereas Andromeda takes you to places that are currently settled by humans or have been very recently. Misidentifying what the core theme of the game should be led them to selecting the wrong technology to base the game around (procedural generation) which was perilous for the game's development.
Another point in which they screwed up right away is in the assertion that they were giving fans what they want because Andromeda is a sequel, and fans gave feedback that they wanted a sequel (for the record, I wanted a sequel as well). The problem is that Andromeda isn't really a sequel because it does pretty much nothing to follow up what happened in the original trilogy. Instead, Mass Effect: Andromeda is what my good friend Mr. Plinkett at redlettermedia calls a "soft reboot" (he goes into depth on the soft reboot issue in his recent Star Wars and Star Trek reviews). They didn't want the pressure of having to carry on the franchise in the Milky Way and crafting a story of the galaxy in the aftermath of the Reaper War, so they created a clean slate by moving the franchise to a new galaxy where they wouldn't have to worry about previous events as much. Personally, I would much prefer a prequel to a soft-reboot like Andromeda.
Since we're on this forum, we all probably have at least some belief that procedural generation can help create a fun game, however, I think most of us would agree that it doesn't work in every context. I think Mass Effect is not the kind of game that it works well for. Any time you are going to be giving your environment close scrutiny and where there is an expectation that environments be very different from each other, procedural generation seems to not be a great solution. So, if you're going to be flying around space, procedural generation works, but if you're going to be walking around planets, probably not. Sticking with procedural generation so long seems to be one of the nails in the coffin for Andromeda as it prevented development of the environments that ended up in the game and it held back story development.
Lastly, it seems that failure to have strong leadership from the start was a major issue. From this article, it seems clear that this game was getting pulled in many different directions and had little direction until Mac Walters came on board. Personally, I think that Mac Walters isn't a great director, but I don't think anyone could have salvaged what he was given with the amount of time he was given (why wasn't this game further delayed?). Giving this game to such a inexperienced team was a massive mistake. Mass Effect fans can be absolutely ruthless with criticism (I hate this about us, I think the ME3 ending thing was overblown, but much of the ME:A criticism is legitimate, even if it isn't said in the best fashion) so this game should not be handed to people who can't handle the pressure. If the goal was to create a game that was the successor to Mass Effect 1, why didn't they bring back many of the same people who made the original game so beloved? I would have done whatever it took to keep Casey Hudson on as director and bring Drew Karpyshyn back in as writer, it would have been worth it in the end to provide more direction to development.