(Warning: Spoilers for the first half of Mass Effect 3)
Western role-playing games may have overarching plots that drive their quest structure, but those plots rarely have any sense of urgency until the very end. In Fallout 3 we're looking for our father but have no sense he's about to die somewhere such that the main quest line requires our immediate attention. In Dragon Age: Origins we know the evil Darkspawn horde is eventually going to attack the capital city of Denerim, but it takes time for an army to march which leaves us plenty of space for side-questing. Mass Effect 3 breaks the mold by opening with the Reaper attack on Earth.
Mass Effect 1 established the Reapers as Lovecraftian horrors that will snuff out almost all sentient life and civilization in the galaxy upon their return. The opening of Mass Effect 3 manifests that horror and provides motivation for the player to rally the other races of the galaxy to the war effort. The opening also provides a narrative push to get related missions done quickly by establishing a sense of urgency. I bought into that sense of urgency and in doing so completely screwed myself out of a ton of game content.
Shortly after the attack on Earth, Commander Shepard has to broker an alliance between the turian and krogan races. Mass Effect 3 goes out of its way to reinforce the urgency of this quest line: The threat of turian extinction is raised during the conference that establishes the parameters of the alliance, and having seen the turian homeworld of Palaven burning from orbit, the player knows that "extinction" isn't hyperbole. Commander Shepard overhears intelligence reports that the turians are close to folding even though they have the biggest military in the galaxy. Admiral Andersen sends updates to Shepard about how badly things are progressing on Earth, and by way of reinforcing the horrors on both planets, the Normandy's Doctor Chakwas can be overheard saying "As we sit here enjoying our coffee, how many souls are in agony this very moment? Millions? Billions?"
During the course of the turian/krogan quests, my Commander Shepard received emails from two major characters from Mass Effect 2, Miranda and Thane, asking Shepard to visit them on the Citadel. Considering I was running missions to prevent the extinction of all life in the galaxy by way of getting the turians and krogan on board with the war effort, stopping by the Citadel to have tea with old friends sounded ridiculous. It felt more in keeping with the story to wait until after the turian/krogan alliance was in place to return to the Citadel.
When I finally got back to the Citadel, it was under attack by Cerberus. After the attack was defeated I found no sign of Miranda in the Normandy docking bay where I was supposed to meet her. I couldn't find Thane, either - but I did find his name etched into the war memorial on the crew deck of the Normandy which celebrated names of the dead. That's when I reloaded an old save game just before I had begun the turian/krogan quest line and went straight to the Citadel to see what I'd missed.
My visits with Miranda and Thane were the least of it. I discovered a side quest that reunited me with Kasumi, another character from Mass Effect 2. Other side quests provided War Assets that would factor into getting the "best" ending for Mass Effect 3. Encounters with Normandy crew members in the Citadel lent valuable texture to their characters and the story. It was enough content that I decided to keep going from that old save, even though it meant replaying the entire nine-plus hours of the turian/krogan quest line I had just finished.