Well the main difference is obviously while islam is absolutely monotheistic, with a far-away god that is inscrutable, innefable, uknowable, therefore unable to be represented in any way whatsoever (this is why it is forbidden to represent it, for example), where the only exception to this absolute gap between Him and us is by the connection he made with the prophet Mohammed, Christianity is all about the Trinity. This means that in Christianity, God is also this "unknowable" monotheistic absolute but with a difference: he made the choice to become human like us, to suffer with us, to struggle with us, to die with us. He even becomes an atheist at the very end ("My God, My God, why have you forsaken me?",Matthew 27:46)... Then, when God dies at the cross, the holy spirit takes over. This is the idea that Christ developed, where he stated that whenever any of his followers were gathered in his name, "he" would be present with them. This is the Holy Spirit and the basis, among many other things, for the existence of the Church itself. (One could even say that it is the same kind of idea that drives other kinds of "communions", yes even communistic gatherings with "comrades", etc.)
In this sense, "God" in christianity is the social sharing of the struggle against the human condition. There is nothing like this in Islam. Islam means "submission", where God is just way up there in infinity demanding full obedience from His slaves believers. They call themselves "Christianity 2.0" but I personally have a preference on the christian theology.