Sometimes I think it's the creative imagination earlier in a piece which causes the problem. Shows like Lost clearly wrote episodes early on which asked really interesting questions, but since the writers didn't know the answers to those questions there was no core narrative which they could tie it all together with. It's no surprise that the deus ex machina they eventually used to tie up all the disparate plot threads was disappointing.

It is interesting how cultural variations lead to differing satisfaction. Western audiences tend to want all plot threads resolved, and the closer together those resolutions occur, the more satisfying they are. Of course, that does result in problems when sequels are created using characters whose primary motivation has been resolved at the end of the original story, as it can be difficult to create a fresh motivation to drive a new character arc without it feeling shoehorned in.

Speaking of Stephen King, the ending of the Dark Tower series was infuriating. I enjoyed the books, but for the author to end the series, then insert an author's note saying "As far as I'm concerned, that's the end. But if you insist on knowing what's inside the tower, here's another ending!" was ruinous. Did I want to know what was inside the tower? No. That's not the point of a grail quest; it's the journey that matters, and the final reveal could never live up to the weight of seven large novels-worth of expectation.

But can I genuinely put down the last book of a seven-book series with twenty pages unread and say "I'm done", and never wonder what might lie within those pages? Unfortunately no. My brain isn't wired that way.

It is interesting that writers often seem to struggle to produce original and interesting endings, even if the beginning and middle of the story is drenched in creative imagination.

Interestingly, what makes a good ending does seem to have some cultural bias in it. Many traditional Japanese stories, for example, are often just a snapshot into the characters' lives, and aren't meant to provide the neatly gift-wrapped endings that Westerners often expect.