Personally, I think Shamus is right about the whole "story not adding up" but, as are the folks who talk about lore retcons. I also believe that these concerns are frankly immaterial. Mass Effect 2 is carried by interpersonal interaction - whether it is moments of Shepard doing (anti)heroic things or getting cozy with the Normandy's crew. The overreaching plot is a vast and distant thing, a story retold many times in various formats with slight variation, but the little moments are soundbites of Shepard's adventure are close and memorable. What do you think is more exciting - a discussion of biotics as explained by ME1 and why ME2 violated established canon, or a compilation of Shepard Badass Moments? Or should the better question be what can excite a larger audience?
As evidence for my case I present to you Avatar, the AAA blockbuster as evidence that for the salesman, what story you tell is less important than how you tell it.