There's one thing that the Reapers didn't count on: giant indigenous sand worms. The big evil alien race that's threatening to overwhelm the galaxy will be very familiar with Shepard after the end of the second game, but the big evil baddies have never read Frank Herbert's Dune . Perhaps that is the weakness the allied races will use to defeat the Reaper threat. It looks like the third game will have set pieces on a scale that could rival the spectacular final acts of the first two games. Roll on March 6 2012.
Update: Commenter Crossngen correctly identifies the "giant worm thing" as a Thresher Maw, which I now remember fighting alongside Grunt in Mass Effect 2, though I don't remember it being quite that huge, or especially hard to defeat. Maybe those Reapers aren't so hard after all, eh?