This is a great question because it makes us stop and take stalk in what values we place on our religions. Can you be a Buddhist and yet not follow the religion -- same for Christians and Christianity and beyond. I would say that I am comfortable being a Christian. And I spend time looking for similarities in other religions.
One similarity that religions share is a common bond that some misuse the notions contained in a religion to do some really evil nasty stuff. Another thing religions share is the fact that there religions are maligned by other religions and people. And for sure all religions share the common misconceived notion that religions are bad.
So that brings us to what is the biggest difference. The biggest single difference in all religions is who they perceive as God. That is the difference. Too many religions to tackle all the different Gods, but one thing is certain they are all different. Of course I do not mean this from a God point of view. I mean this from a man point of view. We color God different and we make God different in form and we shape God to our understanding. And we all have a different understanding.
Perhaps we could say that the biggest difference in religion is the difference between men.