I'm sure deaths like the Rookie or Black Team are going to be a common answer due to the controversy surrounding their deaths.
I know people often defend New Blood's choice of killing off the Rookie by saying that the dude is just a blank slate for the player to inhabit, but I think the Rookie's death, Mickey's subsequent betrayal, and the complete removal of Buck's proposal to Dare all play into how insulting New Blood feels to anyone who enjoyed ODST. It's hard not to feel like a game you enjoyed is retroactively tainted when you now learn that the Rookie will die unceremoniously in a few years time, Mickey will turn traitor, and Buck and Dare will be robbed of the happy ending promised to them by the story of ODST.
It's ironic, in a way. If fans didn't clamor for Alpha-Nine to return, then we would have had to been content with assuming that Buck and Dare were happily married, Dutch returned to his wife, and Romeo would probably continue to hit on women no matter how much trouble he got himself into. And the Rookie would probably be off on subsequent adventures that could only ever exist in the fans' imagination.
But since fans couldn't let Alpha-Nine go, the squad ended up being torn to shreds for the sake of explaining why Buck was all by himself in Osiris with no one else from Alpha-Nine in sight. It is then funny how following Halo 5's reception, Bad Blood does everything in its power to undo the events of New Blood, and reform Alpha-Nine. Barring bringing the Rookie back to life, of course. Although some could say that Mickey being allowed back into the UNSC and Buck's good graces after everything he's done is equally as ridiculous.