Mass Effect 2 still fully embraces its RPG roots, and Bioware does an excellent job of still giving your character a sense of empowerment as your decisions help to shape the way the story unfolds. If you have saves from the first Mass Effect game, you can import them into the sequel and your decisions from that game impact what happens here, which is a feature I cannot say enough good things about. While the first title only has a couple of pieces of so-so DLC, Mass Effect 2 has quite a bit more and while it varies in quality from 'okay' to 'excellent', I find that it all helps add to the overall storyline, which I have become quite invested in by now.
There are some key differences between the first Mass Effect and Mass Effect 2. Most of them come down to pacing. While Bioware tried to update Dragon Age so its sequel was more action oriented, the results were considered streamlined and largely unsuccessful. Here Mass Effect 2 benefits from a tighter focus on combat and menu navigation, and I think the sequel benefits mightily from it.