While Bioware had already struck gold with Knights of the Old Republic, it's only during this recent console generation that the developer's dominance over RPGs has become apparent and Mass Effect 2 is a prime example of this.
Mass Effect 2 takes everything that's great about RPGs (Story, characters, stat building, variety) and bolsters it.
Mass Effect's story had an incredible roadblock to overcome; the original ME identified the Reapers as the main villain while ME 3 will highlight the climax of the conflict. ME2 . . . is almost like assessing the seriousness of the threat of the Reapers. Not a great story to work with.
ME2's response to that is what I like to call the "Watchmen Approach"; the main plot still drives the story along but most of the juicy bits involve the characters, their back stories, and making a bleak world come to life. And it WORKS. Players are compelled to explore every dialogue option with their crewmates and anyone they meet because of how much depth everyone you talk to has. ME2's characters are so good that you're usually too busy delving into their pasts and psyche's to missing the one's from the original and that's quite a feat.
Part of what bogged down ME1 at parts times the absolutely labyrinthine inventory and upgrade systems. ME2 fixes it by making sure there are only 2 or 3 weapons for each class which can be upgraded at a separate terminal using resources you find. Also, only powers and health can be powered up and only four levels at that, making upgrading much simpler and easy to manage.
Much cleaner eh? Literally half the tediousness of the first game is eliminated due to this change.
The second half is gone with the elimination of vehicle sections. To compensate, the side-quest and main-quest worlds are made much larger and varied with a heavy emphasis on exploration so that the Mass Effect world still feels big.
ME2's gameplay manages to mix the fast pace, adrenaline pumping action of real-time combat while adding in the role playing aspect of the rock-paper-scissors advantage system and stat building. Combining this with the unbelievable story and characters, and you have an RPG that stands head and shoulders above all others.
This is the RPG that games on ANY console must measure up to, not just on the 360.