I was disappointed with ME2's ending, but apparently not for the same reason as many of the people here. There's been some bemoaning of the loss of an overarching antagonist, no villian to personally vie with like we vied with Saren in the first game. By comparison, the Collectors are boring and there is no dramatic confrontation with Harbinger. But that's the point. This is the middle of the trilogy, and any antagonist would be a throwaway, between the two developed foes.
Mass Effect 2 is a game about your squad, its about developing them as characters, its about finding out what makes them tick, their fears and hopes, their weaknesses and strengths. Not just as soldiers, but as people. What they believe and why they are motivated are far more important than how many points they sunk into whichever skill. I loved the recruitment and loyalty quests, and how they made the characters more than just gun-toting minions, how each character was unique in a way that mere gun's couldn't differentiate.
Why, then, does the final mission come down to multiple choice competency checks? Despite Jack's outburst in the planning session, picking Miranda has absolutely no adverse consequences; and Tali and Legion never come into real conflict with each other whether or not you've talked to them. The whole draw of a Dirty Dozen, to me, is seeing what happens when you have not only the external but internal conflict dynamic. That's where deaths should have come from - sure kill somebody off when a biotic is picked to go hacking, but you should also kill somebody off when Jack decides she's had enough of the cheerleader and decides to take the opportunity to plant a bullet between Miranda's shoulder blades. When a team consisting of Legion, Tali, and Garrus gets cut off and surrounded, Tali should become absolutely convinced that Legion has betrayed the team.
The final mission takes all of that wonderful character development that made up the first 90% of the game and throws it out the window. There is no sense of culmination, no sense that the final segment is what you were preparing for the entire game. There is a distinct lack of closure, as well. Compare: in Mass Effect 1, you spend at least half of your playtime in the game doing story-missions, or fighting geth in one form or another. Implicitly, you spend that time tracking Saren or foiling his plans. And that makes your final confrontation all the sweeter, ever bullet fired, every gun equipped, every geth splattered and shield shattered has been leading up to that moment.
So, coming after that we would expect Mass Effect 2 to have the same sort of culmination. I mean, can you imagine The Dirty Dozen without Maggot's betrayal? Would you have enjoyed the Magnificent Seven without Lee overcoming his self-doubt? Or Saving Private Ryan without Upham's execution of the POW? The moments that make these films aren't all good moments, many of them cause shock, dismay, and disgust. But they are character defining moments. They are we know that a character has grown, or conversely not grown. Garrus - plagued with self-doubt as to his leadership abilities. Not tactically, but as a leader of people. Jacob has to be shaken by the revelation of how easy it was for his father to slip into monstrosity, has to wonder if the same could happen to him. Mordin struggles with the split between his rationality and his conscience. I could go on, some team member's don't struggle with internal demons but external ones, Tali vs. Legion and Miranda vs. Jack coming to mind. Some even have multiple struggles, as Jacob's greeting of Thane shows. You understand my point I think. The conflict is there, already.
Even sadder is the fact that it shouldn't have been too difficult to add this sort of thing to suicide mission. The classic thing to do would be to split the group up into teams of three or pairs, to take care of different objectives. I understand the dev's aversion to long-drawn out cutscene's in which the player has no interaction. In fact, I was honestly expecting segments where you play as your team members, which would have happened concurrently. (Ergo finish a Shepard segment. Rewind 15 minutes, do the Fireteam segment. Then, in a cutscene, have them meet up again and move onto the next part). Like a movie. I mean we already played a segment as Joker, so why not? Furthermore, what's the point of having a bunch of semi-optional specialists if there aren't optional objectives for those people to complete?
Imagine: After securing a beachhead on the collector base, Tali notes that the base's security control room is not that far off. Shepard has the choice of sending a team there to disrupt the collectors movements and aid his own (like EDI's opening and closing of doors in the derelict ship mission.) If he sends just Tali, however, she needs extra time and won't be available for the next few segments, so perhaps it might behoove him to send Legion as well. Or neither, if he just wants the thing smashed. And so there's the clear conflict setup: does he think that they've overcome their differences? Is it worth the risk?
Say he does send Tali and Legion there, with Garrus to escort them. On the way there, however they find themselves flanked by two large collector forces. The collectors aren't aware of them, yet, but neither can they move. Tali's first instinct is that Legion set them up, and draws on him. Garrus remembers being betrayed himself... and from that point on what happens is determined by how the player resolved the various sidequests. If Garrus' loyalty was never done, he immdiately thinks 'He won't get away this time' and executes Legion. If Garrus was resolved renegade, then he knocks Tali's gun out of the way, and promises her coldly that if Legion has indeed betrayed them that he'll make sure that Legion dies in the geth equivalent of pain. Or, if it was resolved paragon, then he might try to talk her down, reminding her that Legion has nothing to gain through betrayal. Tali's loyalty, on the other hand, determines how well she responds to Garrus trying to talk her down (notice that this is an occasion where playing a paragon would have actually made things significantly harder, but perhaps more satisfying). And Legion, for his own part, might have to re-evaluate and try to quickdraw on the two. The possiblities aren't limitless - quite the opposite, really. But the point is that it dramatically and conclusively resolves the character arcs for this game. Maybe things will pop up in the next game, maybe not, but for now - things are good. And as another benefit, one might imagine this resolution, this closure would have payoff for the characters, either in stats, or future objectives. (IE Garrus might gain a massive boost to his leadership abilities for the Hold-the-Line segment, accompanied by a short cutscene of him walking down the line, snapping orders, and sniping collectors like it ain't no thang. Unnecessary? Yeah. Awesome? Hell to the yeah.)
Some might argue that this character resolution could happen in ME3, but it really... can't. It doesn't fit into the narrative structure. The first act introduces the major players, the protagonist, the antagonist, the world, and the conflict. The third act focuses on the resolution of that conflict, primarily by focusing on the actions of the protagonist and antagonist. The second act is the bridge between those two and it serves to make the heroes victory not only possible, but inevitable. Not because of new guns or bombs or tech or strategy. But because the heroes, as characters, have been developed to the point where we can accept that they can do anything. It's not about power, its about conviction. Shepard already has that conviction, we saw it in the first game when he talked Saren into suicide through sheer strength of character. Mass Effect should have given that same conviction to the side-characters, at least the ones who survived their trial by fire. If Jack and Miranda can stand together, who in the galaxy could stand against them? But now we're into Mass Effect 3. The moment for that has passed, there really isn't narrative room for doing that. It would be like taking the asteroid chase scene from the Empire Strikes back and putting it into the the second Deathstar battle, it throws all of the pacing completely off.
So... yeah. Lot of words. When I first played ME2, I was thinking the problems could be resolved with DLC, but the more I think about it the more I realize that the entire ending needs to be completely reworked, and I know that that's never going to happen, even if its possible. So I guess I'll just hope that ME3 is a solid game, but I'm not gonna get my hopes up about the writing.