Mad Sweeney I think you brought up a good point. I remember as a chrisitian being somewhat confused over this apparent difference of God's personality in the OT and the NT. It has greatly changed in a number of ways. That is exactly way people in the second century, like Marcion, believed the God in the OT was not the one in the NT but were in fact separate individuals.
The Islamic viewpoint in somewhat different from the Christian perception. While we call the Almighty "Ar-Rahman" and "Raheem", which means Most Gracious and Most Merciful, we also call Him:
Al-Mumit: The Destroyer, The Bringer of Death
Ad-Darr: The Distressor, The Harmer, The Afflictor
Al-Hasib: The Bringer of Judgment
Allah has 99 names that describe his being and perfect attributes. Although He loves, He also brings calamity upon the unjust and wicked. All human beings were created with these qualities to some degree but they must be practiced in a balanced way. It is good to love, but it can be bad to love too much to the point of breaking God laws by loving those things that are bad. It is good to hate to some degree, especially those thing that God dislikes. But too much hate can lead to a persons own disaster as well.
The problem with Christianity is that they attach a father-like relationship of God to His creation. The Islamic view is that He is not our father. He is our Creator and Master, who has created us for the sole purpose of worship to Him. Everything in creation obeys his commands, the only exception are humans who have free will to choose submission or not to His will. That is why we are called muslims (One who submits to Allah).
Again, in regards to love and hate, they are both qualities of Allah that he exhibits to perfection and justice.
PVT81