When checking in at the Citadel for the first time, Captain Bailey mentions that "spending a year dead" is a common tax dodge. This is a reference to the book "The Restaurant At The End of the Universe" by Douglas Adams, where a famous band manager is spending a year dead for tax purposes.
During Miranda's loyalty mission, when she and Shepard get on an elevator, she comments why the elevator can't move faster. This is a poke at the elevators on the Citadel in the first Mass Effect, which many fans said seemed to take a very long time to move. In addition, the elevator music from the first game can be briefly heard when Miranda and Shepard first board the elevator.
Throughout the game, the player encounters three different kinds of mechs: FENRIS (the dog-like mech), LOKI (the most common mech), and YMIR (the big and heavily armed mech). Each of these names are allusions to Norse mythology: Fenris the wolf, Loki the deceiver, and Ymir the frost giant.
In a store on Omega you can buy a magazine called Fornax. In the game, the magazine only serves as an improvement to your cabin, and is not readable, but in the world of fan art, it has numerous times been depicted as a sexually explicit magazine. Game director Casey Hudson has said himself that he had always thought the name sounded a little dirty.
When Miranda's character was originally created, she was a blonde of Scandinavian heritage, named Miranda Solheim. When Australian actress Yvonne Strahovski signed on for the role, Miranda's last name was changed to the more Australian name Lawson, and her hair color was changed to black, which matched better with her white and black uniform.
It is the first video game in history where you can import the save game from the previous game therefore the decisions you made in the first game actually have some bearing in the second game (example if a character dies in the first game, he/she will not appear in the second).
The Illusive Man's voice actor Martin Sheen says in the Mass Effect 2 Documentary DVD that he would not trust the Illusive Man. Additionally, in an interview Martin Sheen said that he thought that the Illusive Man was a horrible man.
In the games' Overlord DLC, an announcement is made from an AI in one of the stations: "To all Cerberus personnel, in an effort to reduce workplace stress, music bas been approved for station-wide broadcast." When one listens closely, the elevator music from the first game plays in a more ominous tone, giving a more unsettling feeling.
The trivia item below may give away important plot points.
This is the only game in the trilogy where the player assumes control of a character other than Commander Shepard. When the Collectors attack and board the Normandy, the player takes control of the pilot, Joker, for a short section of gameplay.