Watch the choices you didn't make.
Spoiler note: Mass Effect 3's story is divided into three acts and doles out major moral choices at the conclusion of each. This article discusses these choices in detail, although is written so it can be read in sections. The article's first section discusses the choices made up to and including the mission on the krogan homeworld of Tuchanka. The article's second section details those found at the end of the second act, concerning the quarian-geth war. The final section discusses the game's ending.