It is terribly difficult to offer total freedom to the player -- as well as significant repercussions for their actions, in a video game -- since it involves developing entire segments that the majority of players will probably never see.
It becomes all the more difficult in several games in a row. The Mass Effect saga has tried it, to a certain extent, even if the consequences of certain choices Commander Shepard makes have much less impact than what one would have hoped. In the end the adventure remains rather linear, but you will often have the opportunity to discover characters or messages detailing the long-term effects of your actions.
Plus, all of this is going to help swing the scales one way or the other during the grand finale of Mass Effect 3, with the overall military might of the galaxy.
We are not going to detail here the effects of the hundreds of possible choices during the three games, but rather how to obtain the different endings for each title and their effect on the sequel.
In any case, beware, as there are plenty of spoilers in this article.
Mass Effect 1
Here, things are simple -- the person saved on Vimire does not matter. There is a good ending related to Paragon points, and a "bad" ending related to Renegade points.
Choosing the right ending will give you significant bonus resources in preparation for the end of Mass Effect 3, and it also changes some dialogue throughout different games.
In practice, the impact is not as important as one might have thought.
Good ending: Saving the Council
These three clowns didn't make it easy for you, but by responding to their distress call and protecting their ship, they will survive and humanity will win a seat on the Council.
Shepard will then be asked by the Council whom he prefers to name -- it can be Udina (Renegade) or Anderson (Paragon).
Bad Ending: Let the Council Die
Not worrying about the Council (Renegade) or focusing on Sovereign (Paragon) will result in the death of the current representatives in any case.
This will negatively affect the perception of the human race in subsequent games, as they sacrificed the Council before seizing power. It will also be impossible to meet the Council in subsequent games because of this.
You can choose to appoint human representatives for the new council (Renegade) or a mixture of races (Paragon).