Mass Effect 3 spoilers follow.
The Extended Cut DLC for Mass Effect 3 fixes everything or fixes nothing depending on what issues you might have had with the original endings. If you wanted Commander Shepard to emerge victorious and unscathed that's still not the case, and if you preferred the ambiguity of the original endings you're probably not very happy right now. If all you wanted was to see the gaping plot holes filled in, BioWare handed you a good package. They managed to make the endings smoother and more complete without unduly changing the core ideas that defined them.
Regardless of what anyone thought about the endings in terms of story, there were certain errors that weren't about subjective assessments of story but how the story was told. The original endings were like a paragraph that had missing sentences, yet we were all supposed to understand what had been omitted. How did the Normandy crash-land on a world outside the solar system if it had been fighting the Reapers with the rest of the fleet in orbit around Earth when the mass relays blew up? How did squad mates that had gone with Shepard to fight on the ground in London wind up on board the Normandy such that they walked out of the ship after it crashed?
There were obvious problems with the lore. In the Arrival DLC for Mass Effect 3, BioWare established that destroying a mass relay creates a detonation large enough to destroy an entire star system. So when the Catalyst tells Commander Shepard that all the mass relays will be destroyed as a result of whatever choice she makes at the end, it was reasonable for fans to ask whether Earth was destroyed as well. Mass relays were also the only means of intra-galactic transportation so it actually was kind of astute for some fans to suppose that everyone in the fleets around Earth would starve to death, as they couldn't really go anywhere else and the Earth was a charred cinder.
There were missing transitions and scenes were omitted and that led to confusion and questions that the authors never intended to generate. In an audio interview on the BioWare website, Mass Effect Project Director Casey Hudson openly admits as much:
In some cases, people feared the worst, that, you know, no matter what decisions you make the entire galaxy is destroyed. Everyone starves to death. Which of course wasn't our intention. It wasn't the way we saw it, and it wasn't the intended message about how the Mass Effect universe would end.
I believe him. The Extended Cut is partially an admission that BioWare screwed up, but they did right by us. They filled in the blanks. All the endings work much better as a result. BioWare deserves credit for stepping up and making those changes.
Even more important than these structural fixes is the extended exchange between Commander Shepard and the Catalyst. This scene was far too abrupt in the original ending. We had no idea who the Catalyst was other than a character so important that it was effectively dictating the end of the trilogy. The Extended Cut dialogue between Shepard and the Catalyst provides just the right amount of explanation without pandering to the audience and completely changes the pace of the scene to make it blend into the rest of the game more smoothly. There's even an exchange that addresses what for some people was the problem with the endings.
Everything the Catalyst did, the sole reason for the Reapers' existence, is predicated on addressing the inevitable conflict between synthetic and organic life. It was possible for Mass Effect 3 players to not only make peace between the Geth and the Quarians, but also to rally both species to fight the Reapers. This stark, obvious contradiction of what was presented as the central premise of the entire trilogy is what ruined the ending for a lot of people. In the original ending, the Catalyst presents this as an absolute, and Commander Shepard doesn't even hand-wave at the Geth/Quarian peace if the player made that choice. But the Extended Cut tells us that the Catalyst might be nothing more than a rogue artificial intelligence, not some wise being making statements of truth.