It was clear that the author of Death Note spent a tremendous amount of time thinking about the first few volumes before they published anything. It started densely packed with cleverness, but Naomi Penber was the last fancy trick for quite a while, presumably because they could not continue to invest so much time in each page. There was nothing as intricate and clever as Yagumi Light's hidden desk compartment from that point until, perhaps, the finale.
Oh definitely, the series started out amazing and slowly went downhill to being rather decent. I think that just shows us how hard this kind of writing is, even for really talented writers. It takes a lot of thought and time and cleverness and it's not something that can be just churned out regularly.