I am sicker than a dog, and this is mostly miserable, excepting that I'm getting lots of time to play Mass Effect 2. Holy crap it's good. The Ars Technica review lays down most of the key points, and I'll let that stand. Sometime after I'm done, I really want to get into looking at it next to Dragon Age to compare the respective strengths of these two really fantastic games.
But all that said I want to call out one thing that ME2 does fantastically well: It knows when not to use the system. It's a shooter, but it resists the temptation to turn every possible scenario into a firefight, even obvious ones. Every now and again, you just win, and you do so awesomely (and in a way that really supports who your character is).
This idea of when to throttle back seems to permeate the system, from the barely-there-bones of the RPG system to the new ability to interrupt dialog with action. The net result is something contradictory and pure. The fight sequences are fighty, but the non-fight stuff is engaging storytelling with very little system beyond walking around and talking to things.
All of which is to say, if you GM D&D or some other game where the line between "Fighting" and "Not Fighting" is clear and bright, then you can learn a lot from Mass Effect 2 in terms of how to structure those very seperate things into a strongly functioning whole.
And now, more theraflu.