well sheesh, you did all the work for me, thats basically what I was gonna say... but you already said it all for me... thanks lolWell, part of the problem with Death Note's (anime) ending is that killing L was the bonafide climax of the story. L presented so any more problems to Light than Near or Mellow ever did, even combined. Part of the reason L worked so well is because he dichotomized Light. Light was attractive, sociable, "normal"....L wasn't. It was really played up too, with Light's color of representation being red, and L's being blue. When Light was dealing with L, there was always the chance that He Could Lose. Tension (as a plot device) was extremely high, and as soon as you get rid of L, all of that tension melts away. The fact that Near and Mello were never referenced before they showed up made it really hard to consider them to be the successors of L. Also: Mello was such a stereotype of someone with a chip on his shoulder. That's been presented in anime over and over, it was hard to even care for him. Near was a child genius...still another archetype, hard to care for him. L was an original character, and it felt like his eccentricities were molded onto Near in a way that made him seem like he was just copying off of the original without improving upon it in any significant way, and that's probably the biggest issue with Near. He's a crappy version of a boiled-down L, and it just doesn't work.
One way I think the story could have redeemed itself after killing off L, would have been to actually show Light try and create a crime-free, evil-person-free utopian society. Chances are, it probably wouldn't have worked out, and making Light realize this and/or deal with a psychological fixation on that, or his removal of L could have potentially been more satisfying than introducing Near and Mello.
EDIT: Totally confused who Near and Mello were.