If you love sci-fi and fantasy as much as I do then the cliched-sounding story won't matter because it's told excellently, and the well thought-out world -- especially the alien races -- will get your nerd juices flowing. ME2 introduces new alien characters, while shedding light on some of the alien races that were only touched on in the original. I laughed at the comedic monotone drone of the Elcor, and actually felt uncomfortable with how completely comfortable the blue-skinned Asari were with their sexuality. ME2 also depicts far more instances of interspecies mingling -- particularly in social situations -- that shed a thought-provoking (and often funny) light on intergalactic alien relations. Some of my favorite moments occurred when I took a break from the task at hand just to snoop in on conversations between "background characters." The writing is that good.
A for Atmosphere
But it's not just the writing that makes Mass Effect 2 work so well; It's the strong sense of atmosphere throughout, the distinct synth-heavy sounds in combat and the soothing background themes that keep you company while exploring space. The voice talent is fantastic, but so is the visual delivery. Engage in conversation and you'll find some of the most expressive characters to date -- it's how the characters make eye contact, the gestures they make, the tension you feel when you know that at any moment you might have to act quickly to interrupt the dialogue with a Renegade or Paragon move. And yeah, some of the funnier bits are overacted, but it's intentional -- ME2 often goes for the funny.
Small Changes, Big Differences.
ME2 has all the qualities that we've come to expect from BioWare, but it also benefits from a number of changes that improve it considerably over the original. You no longer gain experience for individual kills, for instance, instead earning your rewards at the end of each "mission" (resulting in a more predictable leveling pace). Other significant, welcomed changes include the all but complete abandonment of loot, the removal of Mako-based ground exploration (instead quests are obtained via planetary scans), and the inclusion of a charming mini-game where you scan planets and send in probes to mine them for resources (it sounds boring, but it's strangely addicting).
I could just say that ME2 improves upon the original in nearly every possible way, but it's far more than that. BioWare took the existing narrative and didn't just continue it, but built it into the framework of this sequel. Games like Quest for Glory and Baldur's Gate tip-toed in these waters years ago, and Dragon Age: Origins showed us real consequences just recently, but ME2 creates true continuity, where every decision truly matters. Mass Effect 3 better be spectacular, because it's going to be tough for BioWare to top this.