The biggest difference - in my opinion
I think the major difference, other than the identity of The Holy Creator of the Universe, is that in Christianity, The Holy Creator of the Universe is reaching down to man - in Islam, The Holy Creator of the Universe, man is trying to reach up to God through works (5 daily prayers, Haadj (spelling?), etc.). Christianity is based entirely on forgiveness, Islam is based on doing more right than wrong. The two views are diametrically opposed.