I thought Death Note, for the most part, was a great manga series. It was the series that got me into manga/anime. Obviously, it was at it's best in the beginning, up to the Youtsuba Kira bit, which for me was Death Note's version of filler, showing what the world would be like had Light not picked up the Death Note. I found that boring. For me, the two best bits of the series were when 1. The second Kira, who turned out to be Misa, had her tapes broadcast on Sakura TV. Way to make an immediate impact. The other great part of the series is where Light has the busjacker touch the Death Note so he flips when he sees Ryuk. As for L's death, I think it was a good way for him to die. His name shouldn't have been revealed because just his name, L, it emphases what an unusual character he is, and allows him to remain mysterious, like he was at the beginning of the series when Lind L. Tailor was killed off. My only problem with his death scene is it was too obvious. You knew that you were going to see L die and there was nothing you could do about it. It was sort of a sinking feeling. A similar thing happens at the end of the first arc of FullMetal Alchemist (I won't spoil it here, spoiler within a spoiler hehe) which really got me down.
Once Light gave his notebook to Yotsuba, I lost enthusiasm for the series. It got too complicated at times. What had been a simple L vs Light battle had changed as loads of minor characters took centre stage for only a couple of chapters, such as Soichiro and Sayu during Sayu's kidnapping and the storming of Mello's hideout. Also Demegawa during the attack on Near's headquarters. My enthusiasm returned with the introduction of Teru Mikami and re-introduction of Kiyomi Takada. Takada's re-introduction came as a huge shock, I found it amazing that Light's representative was somebody close to him, and also Mikami's introduction pretty much let Misa off the hook. It looked like everything was coming together for Light and he would achieve victory and I was excited to see how the final blow to Near would be dealt, or, how the tables would be turned on Light. So imagine my dissapointment when Light declares victory seconds before the SPK is meant to be wiped out. I was saddened because Light's character had suddenly been changed in the moment of truth just so the creators could force an end at 108 chapters. They tried too hard to force an ending in Near's favour and the only way that could be done was to change Light's personality beyond recognition, which ultimately created a really bad ending. If anybody has read How to Read 13, they will understand me.
I think my favourite character in the series was definitely Teru Mikami. The way they introduced him in detail, his background, his motives for supporting Kira, was quite brilliant. Out of all the characters, he belonged in Death Note the most, with the possible exception of Higuchi.
I think L was my second favourite. He was just so unique in the way he did things that it was impossible not to like him. My only gripe about him was that even though he would say that Light was a friend to him, he seemed as if he'd only be satisfied if Light was Kira.
The character I disliked the most was Near. He cheated. And he was obsessed with victory and being No.1. He was a bad winner. L was a liar, but Light could compete with him, and they played mindgames with each other, which was entertaining. Near cut corners, he just assumed things and turned out to be right, of course if he was wrong then the series couldn't end at 108 chapters.
I don't understand why people disliked Misa. She was very loyal and honest about things, and she was a very determined person. She was a bit of a brat sometimes, but I found that quite clever, how a girl who'd lost her parents in tragic circumstances could be so strong, and also so happy.
I think the best idea as far as the notebook goes was the Shinigami Eyes. It always threatened to be an important plot twist, but all it really did was re-inforce the main characters personalities. For example, Light would never trade for the Shinigami eyes, showing that he was determined to have a long and proud reign as the god of the new world, whilst Misa and Mikami admired him so much and would do simply anything to just get a share of that power, in their minds as a reward for helping light, even if it meant that their lives were shortened.
Well, that's my take on the Death Note manga. I'm yet to watch the anime. I probably never will, since it seems to be pretty much the same except from the very end.