I considered approaching each character separately but decided to group the characterization as a whole and cite issues with them if needed.

Overall, the characterization of the characters in Death Note is not focused on and clearly isn't meant to be a huge part of the manga; therefore I don't find the lack of it necessarily an issue in itself. Most of the character development is only seen through the actions of the characters that influence the plot, so as a result most of their background are relatively unknown. I don't have a lot to comment on the characters as a whole so let's move onto the specifics:

All female characters:
Whatever the gender of the writer, Death Note is quite misogynist. I could write a whole essay on this topic but to be brief, all the female characters are supporting characters and are either insipid, insignificant, weak, and the majority of them die . They are also characterized as easy to manipulate/control. Upon further research, this is a common issue in Ohba's works (and manga/anime in general I suppose) so it isn't too surprising but deserves notice.  

Misa:
Ignoring the above issue, I find Misa's devotion to Light is extremely unrealistic. It is highly unlikely someone would cling to a person that is rude, ignores them, and overall treats them in a demeaning way. It's quite clear she is basically a plot device primarily so that Light doesn't need to obtain the Shinigami eyes (among other reasons pertaining the plot) and has a free slave. I also felt like she went through a sort of character derailment in the last half of the series and became even more loud and childish.    

The "stupidity" of the supporting characters:
What irked me was the stupidity of the supporting characters. Death Note divides the characters into an "us and them" type of mentality; either a character is about as smart as Light or they're merely stupid. I found this too convenient and unlikely. Reality dictates that people tend to have a mix of mental capacities and some are stupid at one issue yet very intelligent in another area. Aizawa and Mogi eventually sort of figure Kira out yet it is so late into the series, it makes you wonder why they wouldn't even consider him before in the time between L's death and Kira's reign.

The quality of the "plot" is very divided to me. In terms of events, it was somewhat better than average but it's really the ideas and thoughts of the characters which weave it together. That being said, I have a few plot points I absolutely must discuss:

The memory regaining:
This was not well researched by the writer. Regaining your memory does not equate to being the person you were before the memory loss incident; furthermore, it is unrealistic to have every single memory you ever experienced before the memory loss to consciously circulate within you (however, I excused that because Light does regain his memory through supernatural means). It bothers me incredibly because regaining memories does not mean you suddenly agree with the person you once were; it merely means you remember the memories you once had. You may even disagree with the memories you once had. Given the memory loss Light's friendship with L and his personality, it is unlikely he would immediately pick up his memories and become the Light from before his memory loss. You would think Light would realize this when he decided to lose his memory but once again, the writer's convenient ignorance forgets this fact. I discussed with friend my thoughts on this and he thought that because Light regained his memories supernaturally, maybe he would remember and implement them the same way but I don't logically see that occurring because the death notes states that you will regain your memories, not become the person you were before the memory loss. This is lazy, uneducated writing on the author's part and is probably the most aggravating plot point of the entire manga to me.

Volumes 5 +6:
This is the business Kira arc. While I understand its importance to Death Note, I recall feeling frustrated since it felt like it could be summed up in a paragraph or by narration instead of the reader having to painstakingly watch the whole arc unfold before their eyes. It felt of little importance and could have been skipped so that bothered me. When I was reading it, it felt episodic as well (which in this case is a negative thing since it seemed out of place in this manga).
 
Mello's disappearance:
I found it odd that Mello would seemingly disappear and not contribute as much to finding/getting evidence on Kira as it would seem he would, especially since he has a inferiority complex. It would seem like he would be more proactive in pinning Kira first over Near but I suppose not...(this crosses over with characterization).  

The showdown:
This is purely personal. I would have preferred if it weren't scheduled at one place at one time and I think it would have been more interesting if they eventually killed each other in trying to do the same to the other. However, I completely understand why it wasn't that way, plus it probably would have dragged on if it went the way I preferred.

Light nor L fuck up in their defeats:
It made me feel like they were both too perfect since it is because of another person they lost. You could say it was Light's fault for trusting in Mikami but it felt irritating to me that Light himself didn't actually make a mistake. It is the same with L but to a much lesser degree. Once again, this crosses over with characterization.

The setting is earth with the Shinigami world. I enjoyed the rendition of the Shinigami and their world, especially the character designs of the Shinigami. Nothing too drastic that I would want to comment about regarding the setting in general.  

The art style fit the series well. The semi-realism and sharp black and white images complemented the dark tone the series has. I didn't prefer how the females were drawn and how similar most of them looked to one another but other than that, I thought the art was well drawn.
 

Death Note is relatively fast-paced. There were a few volumes that were particularly slow (Volumes 5, 6, 8, and the start of 9).

My only complaint with pacing was the Mikami background. I felt like it ruined the flow of the series and was frankly, quite tackily executed and didn't add a whole lot to the series or even Mikami. It made me glad the writer didn't attempt character development if it was like this   I had trouble figuring out where to place this bullet    

Overall, Death Note was a pleasure to read. Despite that, the manga gave off the vibe of it being a rough draft that had potential but had many errors that needed correcting before it could be considered a final, polished product. The main error that Death Note contained was that it was too plot driven, which is too general of a word; perhaps the phrase I am looking for is more that the process of thinking behind the manga took center stage and didn't allow other aspects of a theoretically quality manga to stand out. Nearly all the characterization that is displayed only comes from what the plot requires of each character and similarly, the questions Death Note raises about justice, morality, etc. are basically ignored, leaving the reader to decide for him/herself the answers to those questions or ignore them entirely. While the characteristic of a purely "process driven" manga is endearing to some readers and does leave the manga feeling "raw" and real, as a whole Death Note fails to impress me (now, in terms of whether the manga achieved a specific feeling, I would say it did I quality job of that) as a work of art. While I applaud certain aspects of this manga, it left me feeling unfulfilled and almost depressed because of the manga it could have been .    

Final Score: 6.5/10