Sorry, I wasn't precise in my post, I didn't mean "plot-altering" right or wrong, I mean details like an extra conversation (for a few paragorn/renegade points) if you have a teammember surviving ME2, a different sidemission if there is no quarian-geth war, etc. I'm pretty sure it'll be "gather up allies and take back Earth from the reapers" anyway, it's just not the same if the geth are weak, apathetic or completley reprogrammed to follow the reapers (Destroyed, Reprogrammed heretics or didn't activate Legion and sent him away), but in the latter case you might have some advanced weapon against them, which Cerberus developed based on Legion. And in this vein, I think it matters a lot when you try to get the krogan to help whether Wrex is boss or not, whether Grunt is loyal or is kept in a box, or if you can throw in the cure for the genophage to sweeten the deal...
And for the DLCs, I'd like an optional reward for those who completed them (maybe more than a thankyou email like I got for completing Bring Down The Sky), especially for Liara (she should be really helpful) and Kasumi, since they made such a big deal about her box (and a reaper is shown among the images).
Just like in ME2, to complete the game without dying, you need at least two loyal members surviving until the fight against the reaper.
So in ME3, to save the galaxy without for example sacrificeing earth, you'll need three allies, but you can have as many as you'd like (and even more, if you've completed the DLCs)