Because you don't play it for a harsh challenge, the story is pretty decent, but I played it for worldbuilding, art style, ensemble cast, feeling of adventure and journey across a galaxy. That sort of broad feeling stuff.

I really enjoyed the characters. When I think about Mass Effect 3, for example, I think of how I felt when making peace between the Quarians and the Geth, because of how I had gotten to know the characters of Tali and Legion. Or Wrex enthusiastically greeting Shepard as an old friend, something that's only possible if you talk him down in the first game.

I was as disappointed as everyone else at the actual ending to Mass Effect 3, and I do think the plot goes a bit weird even before that (the ending boss fight of mass effect 2 is a bit weird, but again, I think more of the personal stakes that had been set up by good character writing (plus Jack Wall's "Suicide Mission" makes what could've been overly cheesy instead feel grand and epic)), but I found the smaller, interpersonal stories that Mass Effect tells to be quite compelling.