The plot of Mass Effect with the protagonist following that initial mystery, being led through exotic and nightmarish worlds and exhilarating mystical experiences to gradually recognize a seemingly inevitable galactic existential crisis built into the very contradictions at the heart of sentient existence-- accompanied by a nostalgic techno-orchestral soundtrack and the distraction of some sexy characters and sociological case studies--the stimulating brushes with philosophical allegory, the navigation of team dynamics, and the way the game's second and third installments followed the thread while generally improving the interface. (The plot of ME2 is the most structurally perfect, I think.) There are too many specifics--what about the voice actors and the characters they portray? It's obviously difficult for me to assess the confluence of choices that made this series great.
I have a slightly modified quote from Sovereign taped on my office door: "Your civilization is based on [...] our technology. By using it, your society develops along the paths we desire." Many video-games have profound statements incorporated here and there, but this is one that has stuck with me because its pertinence is renewed with every new wave of devices, every system glitch, the "integration" of smart devices into my professional and personal life, the increasingly negative sociopolitical and environmental ramifications of the burgeoning tech industry over the last decade. I'm using their technology. I just spent 40 minutes typing this on a laptop to upload to this GOG website--to what end, really? Am I EDI's Joker? The game gives me a distorted mirror with a compelling (=true?) angle for critique. Maybe it does that for some other people.
So, spectacular elements aside, I think it's the ME story overall that is great, because it has a mythic/parabolistic relevance for me personally that is quite current even though I haven't played it in years. What a game!