The problems with mass effect 3 fall into two main areas:
1) Most of the DLC should have been in the main game because they give us what we were missing and felt like important main story stuff. (From Ashes, Extended Cut, Leviathan, and Citadel's Party DLC's all should have been part of the base package, with a full dlc suite expanding from there that delves into NEW things, not patching mistakes.)
2) Weird Decisions that, from ME3 the Final Hours, could be laid at Casey Hudson's feet. This is the stuff like a big emphasis on the kid, making the ending what it was, and cutting out the final boss fight with The Illusive Man. It seems like originally the ending would have been at Cerberus HQ, according to the art book, and the battle on Earth would have been possibly been a multiple mission planet like Rannoch and Tuchunka (with dlc map Rio serving as an example of content that was built for a different assault on earth.)
3) The Script Leak likely lead to the team scrapping large pieces of content and making something to replace it with that didn't really end up working out in the end. (I think Mac Walters goes into this in the Final Hours and how much it damaged the team and the momentum of the project. If this leak hadn't occurred, I feel confident in saying we would have had a much different final product. I think Tuchunka and Rannoch were least effected, but everything else in the game would have been very different)
The kid just annoyed people and early on it was something Jeff Gerstmann called out as being "fucking lame". Brad Shoemaker of Giant Bomb ended up having a much better experience than the rest of the crew because he had as far as Leviathan when he beat it (So he had from ashes, extended cut, and leviathan. The 3, minus citadel's party, that I said would have benefited the game as being in the base experience) He loved the game and argued for it being on the GotY list and put it at his number 1 slot that year I believe.
These things were missing form the base game and are half of why people got so mad. They didn't find out the ultimate secret of the reapers, the prothean was paid dlc, and the ending was way too short and covered almost nothing, least of all your choices. Adding to that, the kid dreams were sappy and annoying for most people, and you had a mix of art book comments and things Casey Hudson and Mac Walters said in the Final Hours that lead people to believe Casey Hudson had pushed some awkward idealistic thing into the main game that didn't belong and sabotaged the ending.
It didn't suit the themes of the games prior and felt like they came from a guy who didn't eat sleep and breath his product like Hideo Kojima would. Delving into comments he made on the bombcast and in other places, as well as the way he left, he felt like someone who didn't really like the games he was making, and was more concerned with other things that were important to him, but not important to the userbase he had a hand in developing.
It ended up with a final mission that felt thrown together at the last minute and focusing on things that weren't important to the fans but could be seen as important to someone who is looking at the project at a surface level. It was something that looked like "the boss swept in and demanded we change everything" and it's kind of a shame we didnt get a project
(My personal no 1 annoyance was that if any of your squad are missing at the end, such as if tali dies, a random human soldier replaces them in their exact spot despite this being an intimate moment between shepard and his squad. Tali dying is the only one that could be seen as common, as if you didn't get the tali/legion compromise scene in me2 and kept them both alive it's impossible to get both the geth and quarians to survive rannoch, and I personally find the ending where Shepard tries to stop legion short of blowing him away, and then apologizes to tali for not being able to stop things from going this far, ends up in the best emotional story for that part of the game)
But yeah. If we hadn't had the kid, if we had a badass final boss fight, if we had the prothean, improved ending, reaper backstory mission, and additional me2 characters' content (from citadel's party half, including the scenes on the strip, in your appartment, and the party you can throw up to "the night before the final mission", which gives you much more closure with the me2 squadmates and makes their places in the story feel more meaningful) the game would have been much better recieved.
But these things are what I feel dragged it down and their absense lead to one of the biggest cases of blue balls a game has ever left people with. They didn't find out about the reapers, they may have not seen javik (in jeff gerstman's case), they had to deal with a sappy and saccrine plot thread with some dead kid that outstayed it's welcome as soon as the first mission ending, they didn't get a cool boss that put their combat skills and abilities to an ultimate test (let alone the way sovereign amounts to a bucket of nothing after coming face to face with shepard in the final run), and an ending that felt surreal, out of place, and empty. Their choices didn't factor in and it felt like we were just hurried through the experience to make a statement that unfortunately threw the series promise under the bus that we'd believed in for about half a decade.
It was a shame and could have been better. People coming to the game late had a better time than the people who loved the game most, and this game is part of why I started to question dlc practices and the harm they can have on a game, pushing it out when it's not ready with the idea that they can fix problems it has after the fact.
This game was a case where the product we were giving was lacking and the prospect of dlc wasn't exciting new adventures, it was a bandaid for people that cared about the series most. Oh and the original final message about continuing the adventure through dlc was the worst. (Let alone the crappy buzz aldrin thing. Bringing in that guy to do a voice should have been the coolest thing for this specific series, but the way it was used was just the cherry on top of everything people disliked about the game)