Death Note is definitely an outstanding show, full of suspense and engagement, plus the depth they went into with everything, I bet it took them an enormous amount of time to write, and revise all of their ideas. It is truly a show not to overlook, very addicting too ;).
I suppose it could have taken a long time to write. And yet, sometimes the most excellent fiction gets written in the shortest amount of time.
As both a writer and a computer programmer, I often hear about getting "in the zone", as it is a phenomenon that is common to both disciplines. Zone-writing results not only in greater quantities of text in a short amount of time, it also produces more organic and engaging stories.
I get the impression that the writers of Death Note were very adept at zone-writing.