Quote from The New Flesh
When the most interesting character in the series dies and the manga can't seem to figure out how to keep itself interesting without him.
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Death Note
When a manga starts indiscrimately resurrecting characters from the dead for no good reason.
1. when you tag it like that it doesn't handle much.
2. The series went to crap when Misa Amane showed up. not sure why you'd be complaining about things that happen after, unless you were referring to Ray Penbarr's fiancee whose name I can't even RECALL. but after that the series had no spark of goodness in it. saying what you say just means you didn't NOTICE, but really it was over at that point, you were just hoping for a nice end without REM being horrible at extending the plot.
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if REM got L, and Watari was not taken because she turns to dust, and Watari puts a sniper round in Lights head while he's walking down the street thinking he has won, the series would have redeemed itself. felt like thats where it was going, but somehow it got ...extended
3. the ressurection thing sounds odd, what you probably mean is various things that negate progress, your resets in general, amnesia, people being unreasonably dense, the bad guy getting away for the nth time.