If there's anything that will put me off a novel or book series, it's loose ends. I want every question to be answered, and where appropriate I'd really like to learn the ultimate fate of every single character, ideally. And that's why I don't truck with stories that contain unresolved subplots. Here's why it ought to matter to you: I'm not the only reader who feels this way.
But What About the Dog?
Subplots are used to add meat to the main story. They're around to provide excitement, or perhaps mystery, maybe even romance. In the best stories, I find, the subplots are intricately woven around the main plot and all the threads interconnect somehow. When subplots are just hanging out there for no reason, I always feel a bit like my time is being wasted so I appreciate it when everything ties together.
And frankly, I want it tied together in a nice little bow when I get to the end of that story. Even if the end of the story is that everyone's dead now, this is a resolution I can live with. What I can't live with is not knowing.
For example, whatever happened to Leah Clearwater from the Twilight series? I always found myself particularly concerned about her well-being, and hopeful that she would find a love of her own. Stephenie Meyer never gave me that, darn it.
If you close a book and the story's clearly over and you're still asking yourself something like "but what about the dog?" it's an example of an unresolved subplot. And I don't know about you, but it really sticks in my craw.
Always go back and read your book, looking for loose ends. If you still have questions, then so will the readers. Tie it all up for them, and put a bow on it. Because there are some readers out there who won't care about your subplots. For the readers that do, go ahead and put a cap on the story...because it certainly won't hurt those others.