I took my previous tropes entry, combined them, edited them for clarity, and added a few. If you have any more, put 'em in the comments!
Executing Order 66 - Systematically killing off the cast. Fun for slasher stories, not for when you're writing a drama and have no idea how to make it dramatic without death.
When it works: Game of Thrones - We care about the characters and they die with respect to the situation and the results of that death has lasting consequences. The deaths are NOT WORTHLESS.
When it doesn't work: DC's Countdown. Watch Linkara's videos about it to understand why this fails so painfully.
Phantom Menace Challenge - Have