In canon, the explanation is that sacrificing your life in order to save someone else has actual magical force (the events of book 7, in which this effect works for Harry even though he did not actually die, imply that this effect is tied up with a person's intent to sacrifice their life, rather than their actual death). Thus, if a wizard knowingly and willingly sacrifices their life to save that of another, that other person gains a measure of magical defense, which was the reason that Voldemort's first attempt to kill Harry rebounded. I'm not sure how or whether this will be changed in MoR; if you use the intention interpretation, then it doesn't seem to horribly contradict any of the established contra-canon facts.
if you use the intention interpretation
I agree that the intention interpretation seems to be the correct one, based on canon. But that just makes all the more incredible that nobody has discovered this before, that Harry should be the first person in known history to survive the Killing Curse. So, it would be nice if MoR made this more sensible, especially if it could do so without contradicting the rules as they are played out in canon.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: