Mass Effect is a action RPG based in the future, and is about your character trying to protect all organic life form the Reapers, a sentient form of super machine that eliminate all intelligent life in the galaxy every 50,000 years. The most revolutionary thing about this game is that most evrything else is up to you. In game you get to create your own character devolope skills and relationships, all of this is transfered through all 3 games, it really lets you deavelop a character that really resonates with you.
image from: Mass Effect screen shot from flickr
The game makes you make big decisions that will affect your character in the long run, for example you get to choose in the first game whether one or the other of your squad mates dies this will affect who you can play with later on in the game and how some people will react to you. There are also the small moral decisions like whether to help a old man get his daughters body form a military testing lab trying to find out how she died. All of these little decisions ultimately cultivate into the complexity that is your character.