There's some talk about Mass Effect 4.
I remember really liking the first three, although they got worse with each release. I remember liking the characters and the feel of the universe, although it had some transparent flaws that got more glaring with each iteration. The big draw was the characters.
But now, looking back, I don't remember the characters much. I mean, I remember them - Tali and Wrex and Liara and so on - but I don't have any fond memories of them. The only characters I remember well are Chakwas and Garrus.
Because they hung out with me.
Hell, I remember the nameless meat-head marine in the last game better than most of the characters I actually liked, because his first major scene was him hanging out with me. Sure, he was annoying, but we interacted socially.
Game designers are typically worried about bang for the buck, so the scenes they write are usually A) exposition, B) entertainment, and/or C) rewards. For example, Mordin was written for "entertaining" exposition, and even had a few purely entertainment scenes, such as his famous song. On the other hand, Liara is basically the same character written for "rewarding" exposition - she's nice to look at for the target audience, has an attractive voice, wants to date you instantly, and spouts exposition.
All of these options are aimed at the "unnamed player" - the person pushing through the game. Although in theory Liara and Mordin are interacting in-world with Commander Shepard, they're really aiming at the person holding the controller. You can clearly see that by how your responses are framed: Shepard doesn't have any strong or nuanced reactions, just a basic binary response you select from the generic menu. The scenes are aimed at the "unnamed player", not at Shepard, your version of Shepard, or you playing Shepard.
Easy way to tell? Shephard never laughs, grunts, or sighs until you say she should.
By removing Shepard from the equation, it's easy to make generic, highly acceptable events. Shepard has no nuanced response because that maximizes acceptability: if a player finds Mordin's canned, West-Wingish dialog annoying, or hates blue space babes, that's okay. Shepard can hate them, too, just select it from the drop-down list.
That said, it's no mistake that the most memorable scenes in the game don't ask you to judge anything at all. When you're shooting with Garrus, it never asks for your opinion on Garrus or on shooting or whatever. It just asks you whether you want to hit or miss. Shepard is allowed to react as Garrus' friend, and they get some very pleasant and rewarding patter going without the "unnamed player" ever stepping in to pass judgment.
That's why the scene is so powerful: it builds up an easy, rolling pace. The characters are in the right world, behaving as they really would, and it doesn't interrupt itself to ask the player what they think. At this point, if you aren't friends with Garrus, you can screw off.
I don't know if that scene is the thing everyone remembers most from ME3. But after time has passed, it is the first thing I remember about all three games. And I replayed ME1 after ME3.
The Chakwas scene is similar. You aren't asked to judge whether you like Chakwas or not, or what you think of her talking about the old days. Instead, it's allowed to unfold gently, organically, from the two characters. The player is not involved, but because the scene is honest, they'll remember it for a long time. I can't even remember which game the scene was in, but I remember the scene.
Stop making generic protagonists. Stop asking the player what they think.
Let the player choose by doing. If the player keeps approaching Garrus, assume they like him. Give the player choice by giving them physical choice: hit or miss. Drink or leave. Hug or pat. Then let the scenes work out based on the characters. People will remember it longer, I think.