The answer is a lot simpler than most of you make it out to be: There are like two-dozen media sources devoted to reviewing Xbox related games, and they pretty much all automatically give a massive and great game like Mass Effect 2 a 10. PC does not have so many dedicated review sources, and thus doesn't get that extra slant.
And yes, the game plays a little better with a mouse and keyboard, and looks a little better on PC, but overall the versions are nearly identical.