Anyway, player choice. It's one of the most praised aspects of Mass Effect, and deservedly so. Leading into the third game, every player was wondering how the game would change if s/he chose to let Ashley or Kaidan die, to kill or spare Wrex, to destroy or salvage the Collector base, etc. Bioware knew what it was doing here: you needed a reason to return, and what better way than to entice you with seeing the consequences or rewards of your choices, for better or for worse? It wasn't just the big ones, though: whenever I saw a throwback to an earlier game - Rebekah and Michael Petrovsky and Char and Ereba are my favorites - I felt like the developers cared to give me a follow-up, even if I couldn't interact with the NPCs directly.
Obviously, I knew that I was playing a game, but it was easy to just get lost in the character building, even if a great portion of it existed only in my head. I could see relationships - romantic and otherwise - develop, form alliances based on charm or brawn, aimlessly roam the galaxy or get right to business. And it was all at the tip of my fingers, playing a personalized What Would Shepard Do. I felt connected to the bigger picture by Bioware's use of microcosmic elements, like I had a reason to fight for the survival of the known universe. Absolutely no other game has done that for me like Mass Effect, not even its thematic predecessor, Star Wars: Knights of the Old Republic*.
And here's where the bad comes in. There was never any way that Bioware was going to resolve the game to everyone's satisfaction, which was made obvious by the time gamers had reached the finale of Mass Effect 3. Any company that makes something for mass consumption has an understanding that, no matter how much they try, they're going to piss a portion of their consumer base right off. Now, did they have to say that all the choices a player made from the beginning would make a difference? No, they didn't. Because, let's face it. Mass Effect was essentially a very complex "Choose Your Own Adventure" book. Only so many options can exist, unless you expect the creators to publish every single possible outcome, and well, that's not a thing that can actually happen. There are just too many variables.
That being said, Bioware (although I'm going to place the blame on EA, because they are awful) did make promises they weren't ever going to be able to keep. They essentially painted themselves into a corner, and even an extended version of the various endings couldn't appease the majority of complaining fans. I don't know how I would have resolved the series, but it wouldn't have been their way, with an annoying Deus ex Machina character instead of the expected epic battle with Harbinger and his badass Reaper pals. But if choice hadn't played such a large role in the rest of the saga - and hadn't been handled as well as it had - I don't think people would have been as upset about it.
< Day 19: Best Antagonist
* And yes, I do consider KOTOR to be the progenitor of the Mass Effect series. If you look at the two stories, they're fairly similar, at least in the grand scope: main villain thinks that s/he is doing the universe a favor by joining the bad guys, only obviously Shepard doesn't turn out to be Saren ... although that would have been ... interesting.