In the interest of fairness, let's start with:
The Good StuffActors
Bioware has some great writers, and the actors absolutely nail the dialogue. Female Shepherd in particular does a great job. Anyone who has finished ME3 as FemShep knows the beautiful delivery of some lines near the end. The confrontation with Mordin is fantastic.
ME3 is a pretty game.
The combat is fun, but isn't too substantial. A Hide-Behind-The-Barriers game like Gears of War or ME requires some work to be interesting, or it becomes a shooting gallery, only testing the player's patience. Unless you're doing some work to play the game in a more fun manner, it's easy to fall into the shooting gallery feel.
All the good gameplay is just Mass Effect 2, but with different geometry. ME3 is more of a DLC pack for ME2 than its own game.
The Bad StuffKai Leng
Kai Leng may be the lamest character in video game history. I know he's not a super-important part of the game, but he's so annoying that he deserves top billing in this list. He's the sort of character where the writer is so pleased with the "coolness" that they lose all perspective. Like his fellow "cool" douchebag, Deus Ex's Adam Jensen, he has cybernetic sunglasses and delivers his lines in a stupidly "cool" way. He is a samurai because that's "cool". He ends up being a "cool" guy that has no personal agenda or personality.
Also, Kai Leng is Bioware's Achilles heel: The Boss Fight. Every boss fight in Mass Effect is pretty dumb, I think it's because science fiction doesn't lend itself to shooting a guy a billion times while their health slowly goes down. It just doesn't seem right to fire a sniper rifle into a guy's face, and he doesn't react except to jump to his invincibility spot and a helicopter shoots you.
OverallI think the main source of ME3's problems is cheapness. Most of the elements are copied from 2. The graphics, most the characters, and the combat system are recycled.
GameplayThe problem with the gameplay in ME3 is repetition. Where Mass Effect 2 occasionally made you do different things, ME3 is unashamedly a series of killboxes. The biggest variation in a level is whether you have to find a computer and press a button, or just kill everyone to finish the room. Without any locations leading to the killboxes, I got the feeling that I was on a series of islands, separated from any other place or context.
Everything outside of the killboxes is completely underdeveloped. Piloting the ship is a waste of time. You can get resources that you never see, and you can give them to people you can't otherwise interact with. By completing that exciting adventure, you increase an integer that has a marginal effect on the stupid ending.
(Ex. You overhear that a person needs some DNA from a planet. You go to the correct system and randomly scan until you find it. Then you return to the person and give it to them. There is a text message saying that there are now heroes riding dinosaurs fighting against the bad guys. There is no point at which you interact with an enemy, or see any of the results of your actions. Riding a dinosaur into combat is a cool idea, but you'll never see it because it doesn't fit into Mass Effect 2's combat system.)
I think the weakness of the gameplay is best shown by the post-ending experience. Once you've beaten the game, you're kicked back into the game to the point before the final mission. Once you're there, there's nothing to do except do the final mission again. You can fly around in space, scanning for resources, but that's less interesting than minesweeper.
I think the fact that there's nothing to do after the ending is part of the reason people hate the ending so much. They unconsciously associate the disappointment of there being nothing left to do with the disappointment of the lame ending.
"Choices"Mass Effect 3 is full of false choices and quick time events that don't do anything. There's a QTE to save Tali, and it doesn't work either way. At the end of many missions, you make a "choice" that just changes your military score by 5 points more or less, and generates a text message later on.
I think the reason your decisions can't have any effect is because of the way the game is structured. There are so many potential combination of results, there would be millions of potential interactions if they could affect each other. Instead, all the decisions are totally insulated and modular. No matter what you decide on mission #1, mission #2 will unfold the exact same way.
Even the last decision of the game is a non-decision.
DLCThere are two kinds of DLC: Missions, and Equipment. The Equipment packs are horrible. You have to pay real money for a trivial change to combat statistics. The Missions are just new killboxes.
SexismEDI's design is completely ridiculous. I guess they insisted that she look "sexy" so that she could be a potential mate for Joker in one of the dumb potential endings of the game. That argument would be more defensible if EA/Bioware had a single female main character that wasn't sexual. Even Dr. Chakwas has the body of a teenybopper with the head of an adult.
Having Tali's face be a photo of a human woman with a tiny amount of photoshop sure was a terrible idea. (Not a big deal, but there's no way a Quarian face would be mysterious. They fight and die in battles, and it would be trivial for someone to take of their mask. Not to mention Wikipedia and medical scans)
What the hell was Diana Allers' deal? She's a reporter, but she wears a tank top thing? For a game with interesting characters, she's the worst. Except Kai Leng, of course.
What's the point of having all these sexy characters if they're going to have sex wearing underwear? I know the previous Mass Effects got criticized for their sexual content, but that was from idiots who don't play games anyway. I'm not asking for an AO game, but sex is traditionally performed in the nude.
Minor ComplaintsJavik is lame. You spend all this time and effort dreaming of encountering the Protheans, and when you find one, he's just another guy that crouches behind cover between shooting guns.
Background characters have great dialogue, but when it repeats all the time, the illusion is destroyed.
No one has any ability to make a decision. Shepherd has to make every choice, and can't even ask what people think before they do. I tried to make decisions to please people, and half the time I ended up angering them because they secretly didn't want me to do that. Once the decision was made, I couldn't change my mind without reloading a save.
Once you sleep with someone, everyone knows about it instantly. Why can't Shep have some casual sex?
Cerberus turned from an ambiguously evil group to cartoonish supervillainy. It's more interesting to have them have good goals, but bad methods and priorities.
Legion's mission is terribly boring and has negative replay value. Please, Please, Please, let us skip things where the character cannot make any decisions.
Mass Effect seems to think that there is an inherent difference between Artificial Intelligence and Biological Intelligence. There's no reason to think that this is the case. Maybe they saw Battlestar Galactica? Come to think of it, that Robot Vs Human dynamic feels like it was lifted from BSG.
BugsI got stuck several times, requiring me to reboot from a previous save. You can walk off the edge of the platform at the final decision of the game.
The EndingWe all know that Mass Effect's ending is terrible. I wouldn't agree about the reasons that most people cite, but it's not worth getting into.
Putting the ending on Earth was another lazy decision. Surely people understand that the destruction of all intelligent life in the galaxy would also affect Earth? It reminds me of the time in Captain America where the entire East Coast was going to be attacked with WMDs, and the Cap only really cared when he saw that New York was one of the targets. Are audiences really so stupid?
Fine, You Do BetterWhy don't I make a better game? I'm not a multi-million dollar corporation. But I can generate some better ideas. My version of the game would have a level of decisions that involved the entire galaxy as a stage of action.
It's supposed to be a massive war that leverages all the resources of all intelligent species. Why not actually have Shepherd interact with some of those resources? You could assign troops to different regions of space, or hold them back for the final confrontation. Your alliance could spend more ships on a particular region, making it easier to fly and scan in that sector.
You could try to convince a faction to join you, or choose to let them defend themselves against the Reapers on their own. If you really needed something from a group, you could spend ships and resources to conquer them. It would create a tension where you hope to get more resources from the conquest than you spent on conquering them. Different characters would react different ways to those decisions.
Instead of having Shepherd do every trivial stupid mission, you could assign allies to do stuff.