I want the main plot to conclude satisfyingly, but I don’t need every facet explained or character arc to draw to an end. In fact, I like an open-ended character arc, that suggests there are more stories in their future. But I want a murder mystery or alien threat to be dealt with in a non-deus-ex way, or at least in a method I can follow and doesn’t get all mystical and odd.
I recently read a book called Mainspring, by Jay Lake, that had a decent backdrop, and started out reasonably well (though it meandered pointlessly for a while) then halfway through it took an unexpected left turn, and became a rushed and incomplete, and somewhat uncomfortable, mess. The ending was complete rubbish, that made no sense and had no real connection with the original set up.
It was like he had enough ideas for half a book, and then just started to make crap up to meet his deadline. I was very disappointed.
having said that, I know from my limited writing experience that endings can be very difficult to write. Mostly because in the real world, things don’t end in satisfying ways, they just peter out and invisibly blend into other parts of your life.