If it’s part of a series, I like the main plot situations to be tied up fairly succinctly, but it’s fine to have some type of an unexpected cliffhanger or at least some type of situation to lead you to believe that “more is coming.” It doesn’t matter so much whether the ending is happy or sad, as much as that the ending makes sense. The ending can be a surprise, but it has to actually make logical sense with everything else that went before it, even if I didn’t see it coming, otherwise it seems like a cop-out. But, happy endings are much more enjoyable, I suppose. I guess bittersweet endings are good too, but a sad ending that comes out of nowhere, just for the shock value is bad.
What I hate is when books (or movies) just end and there’s no resolution, no idea about what actually happened or no idea about what could happen. I’ve noticed that a lot of foreign films that are pretty interesting all the way through, until the end, end by just stopping. I’m often left feeling gypped or thinking that I just wasted 2 hours of my life.