Mass Effect 3/Previously

Importing Saves
Xbox360, PC and PS3 presumes the choices you made from Mass Effect 1 and 2 has a continuing effect on this game. But if you're new to the game, the choices is default to either a man or woman. The Wii U is the exception to the rule because of the addition of the backstory to bring you up to speed.

Here is the list of the choices you made so far if you decided to start fresh without any input from earlier games. Also the choice on the Wii U back comic determines who survives the suicide mission.

Mass Effect

 * Garrus is recruited.
 * Wrex is recruited.
 * The colony on Feros was not saved.
 * Shiala, the asari taken by the Thorian on Feros was killed.
 * The rachni queen on Noveria was killed.
 * Shepard did not have any romance.
 * The Council died and the Destiny Ascension was destroyed.
 * Udina was named Councilor.
 * Side missions and downloadable content was not completed under the assumption of sticking to the story.
 * Male Shepard save Ashley Williams and Female Shepard save Kaiden Alenko.

Mass Effect 2

 * Shepard didn't complete Mordin's loyalty mission to either save or destroy the genophage cure.
 * Tali was exiled by her people.
 * Shepard don't have any romance.
 * Zaeed and Kasumi was never recruited.
 * Mordin is alive.
 * Garrus is alive.
 * Grunt was never woken up from the tank and is never mentioned.
 * Jack died during the suicide mission.
 * Tali was recruited and alive.
 * Tali never romanced.
 * Samara was never recruited. She fought her way off of Illium and was later killed by Morinth.
 * Thane died during the suicide mission.
 * Miranda is alive.
 * Jacob is alive.
 * Legion was never reactivated and does not have the name Legion as a result.
 * Shepard destroyed the Collector base rather than turn it over to Cerberus.
 * The only Normandy crew members who survived the suicide mission were Joker and Chakwas, Yeoman Chambers, Ken, Gabby and all other Mass Effect 2 non-squad crew are dead.
 * Side missions and downloadable content was not completed under the assumption of sticking to the story.
 * Liara becomes the Shadow Broker.

Special note: The Wii U version allows you to choose how you want it to happen. Although it's optional, you can choose how you want Mass Effect 3 to play out by the deciding factors above on only the key story points and love interests which set the outcome. For example: Saving council, killing Wrex, Save Ashley etc. However, you can't get to decide on loyalty missions and side quests leaving the outcome on default, presuming that you're only interest in finishing the story.

Wii U Special Edition
For the sake for people who is playing on the Wii U, there will be a list on what choices impacts prior to Mass Effect 3 storyline, especially if you're a male or female Shepard:
 * Love interest to Ash/Kaiden or Liara or focus on the mission. This plays a main role in the person you fall in love with.
 * Save or destroy the Rachni queen. If you kill the Rachni queen, the reaper made version of the giant spider would be replaced.
 * Save or kill Wrex. Wreav will take over if you do kill Wrex (as with many other krogans).
 * Save Ash or Kaiden. You can not have both. Ideally Male Shepard=Ashley Female Shepard=Kaidan in terms of romance.
 * Save or sacrifice Destiny Ascension. The councilors in the Citadel will mention the replacement of humans if you did sacrifice the Destiny Ascension.
 * Anderson or Udina became councilor. Simply who the person you agree with.
 * Get more or less team of squads. Hardly any difference in terms of the people you were working with. Less team of squads will have the love interest of Miranda and Jacob for both male and female Shepard.
 * Resolve or ignore conflicts within the team of squad. Ignore conflicts can easily decimate the squad team.
 * Stay true to your friends or move on. Catch up with your old friends after the game or continue with your mission after the Collector mission.
 * Save or sacrifice the crew. Save the crew will have additional people like Kelly Chambers. Sacrificing the crew will instead have people like Jack, Thane and Legion.
 * Male Shepard can find comfort in Tali or Miranda. Female Shepard can find comfort in Garrus or Jacob. Both can also focus on the mission. Add another person to have a love conflict to this game.
 * Save or destroy the Collector Base. The choice determines the Illusive Man response on Mars Attack.