The free, 1.9 GB update is simply to address complaints over the original ending of the game. It wasn't so much a gameplay complaint, but rather dissatisfaction over the game's narrative conclusion.
The patches add "additional cinematic sequences and epilogue scenes" which add "deeper insight to Commander Shepard’s journey based on player choices during the war against the Reapers."
Mass Effect 3 was released in early March, which means it's taken BioWare more than three months to fix something that shouldn't have been an issue from the start.
Note: there is a new final option where you "refuse" the Starchild's options, or even shoot him, if you'd like, and you "lose."