I'm sure there will be specific endings for people like us who have played through both games at least once. And I'm sure there will be an ending where you fail, or one where at least Shepard dies, like in ME2. But saving the rachni queen, sparing Wrex, overwriting the geth, saving the Collector base, etc., won't be required to beat the Reapers. Like I said, they're going to want new gamers to be able to win even though they've only just started with ME3. I wouldn't be surprised if there's a specific ending where doing all the stuff in ME1 and ME2 makes the victory easier/more difficult, though.