Stories that don't really end so much as 'meander to a stop' or 'lay the out the stops on the next adventure' VEX me.
That's also one of the reasons why I think I enjoy the MCU, anime and BBC shows so much. They often draw from other already established/ongoing sources (i.e. comics, manga, novel series, etc.) and can take literal years to produce between instalments. So those other mediums have the luxury of condensing the narrative into stand-alone arcs within a much larger saga with their own beginnings, middles, and ends.
Nothing can make me more aggrivated when finishing a story than to read that a few seconds after defeating the Big Bad, some completely new and completely unforeshadowed character appears out of nowhere saying that "The real battle has only begun".
When you look at the first Iron Man movie, Tony Stark saying "I am Iron Man" is a proper ending. In the post-credits scene, Nick Fury doesn't come out of nowhere. He's Coulson's boss.
Yeah, that makes sense.
If we're writing, then we want somewhere between bittersweet and happy.
If we're reading, then we love tragedy.