The Old Testament is what it is, a record of mankind's growth and enlightenment towards God and his plan for us. Gods plan for the salvation of mankind did not happen overnight it was a gradual process. It is also an historical record of the times. It contains the foundation of the Christian faith and we are to learn from the lessons of examples given of the Biblical Patriarchs of that time. You may see your Quran as an instruction manual to be followed verbatim, but then it was formed in a very short period of time 22 years as opposed to a period of 1500 years it took to formulate the Bible. The two are not really comparable in many ways.

The New Testament was the culmination of Gods work. :)