So, without spoiling, Mass Effect 3 tracks what you do. Like, "you harvested a dragon for its scales to make armor, +50 points" kind of thing.
Its relatively organic, but of course the people who do all the sidequests will get more points.
And, here's the kicker, to get the "best" ending, you need to have a certain number of points.
In multiplayer, you basically have different soldiers you can use. If any of them reach maximum level (which isn't particularly high... I think it's 20? Maybe 10, Haven't played in a while) you have the option of trading them in for a hunk of single-player points.
I don't mind it, to be honest- it lets people who don't want to play every sidequest still see the best ending, and it encourages replayability in multiplayer. And if you don't care about multiplayer, I'm pretty sure you can hit the required number of points just by playing single player.
(Also: Keep your save games. It is SO much more satisfying to import a save game)