I feel the Mass Effect team is slowly stumbling in the direction of using game mechanics that best fit the story being told. It is fascinating to watch them do so from such a different direction from my own. I was raised, to an extent, by Tim Schaefer's experiments in this direction and Double Fine is certainly the model for me in terms of choosing game mechanics that fit a world. It is silly that gaming has named its genres after collections of mechanics and systems, with the boxing constraints that entails, rather than after anything more meaningful.
Mass Effect comes from a pinnacle of Western RPG orthodoxy. I respect the talent and skill represented in Bioware's catalog. I've bought many of their games over the years, and yet Mass Effect games are thus far the only ones I've beaten. I even beat them in back to back marathons, an altar offering to a gaming god as yet unnamed that gives me some small recharge in my interest in possibly working in the industry. (Hire me.)
As someone else pointed out, it has been useful to throw Dragon Age into the mix for added perspective. Dragon Age is very much somewhere near the apex of the orthodox western RPG. It's a fascinating collection of systems, certainly, but one that more often than not distances me from the games.
Mass Effect, the first, pared down the required systems, and Mass Effect 2 only continues the surgery, trying to focus on the meaningful stories and necessary systems. It's a fascinating progression that belies the traditional nature of game sequels as generally more complex over time. Mass Effect 2 is easily my favorite Bioware game to date, given how much it throws out and re-evaluates as much as anything that it does leave in. (There's a Zen koan in that, of course.)
I have a few minor nitpicks: I liked the elevators of the first game, and I missed the Mako and wouldn't have minded at least one interesting Mako outing. My biggest disappointed was the small weapons tech retcon that was used in an attempt to avoid a larger retcon. I have thoughts on how it might have been done better, but I'll omit them from this long enough post. I also thought that certain plot themes were odd in their under-developed state--- the parallels between Saren's undoing and Shepard's path in this game. I assume that some of that will come up in the third game, of course.
Mostly I spent Mass Effect 2 grinning. From every random cameo, to feeling like a badass with magic powers, it really felt like this galaxy belonged to my Shepard, and that's what I like to play. I was amused to find the story of Blastos, both in game and out. There are some surprisingly funny moments, some surprisingly touching moments, and there's not much more to want from a story. (Although I wouldn't mind a surprise twist that places me to work on ME3, if anyone has any Bioware connections...)