I read a lot of fantasy, where nearly every story is clearly open for a sequel, or is part of a trilogy, and it's not all that unusual for major plot elements to carry over to the next installment.
My personal preference is that no major character be in mortal danger or face an uncertain fate at the end of a book (i.e. no cliffhangers), and that whatever question or goal that drove the min character forward for most of the book be resolved to some satisfaction, even if it's created a new one that will obviously carry you into the next book in the trilogy or series.
So it's all right by me they've fought off the invading army or found the map or rescued the heir to the throne or foiled the plot, but doing so has clearly only earned them a breather or provided them with a new goal.
But my preferences here seem to be increasingly ignored by many fantasy authors. Just finished a debut novel where one of the protagonists was in a coma at the end.
When a series ends completely, though, I like there to be some kind of end to all the little things that seemed important, even if they're not the main plot. I don't like it when the fate of even minor characters remains unclear, if there was something about that character that attracted my attention and sympathy as a reader at some point. Even having the other characters wonder about it or express regret that the poor dog evidently didn't make it out of the burning building, that brings a sense of closure more than it seeming like they (and therefore the author) completely forgot about it.
I can't remember the story now, but I remember one where there was this interesting, though minor, character who popped up a few times, then disappeared. And I kept expecting them to show up later, but they never did.
