If we can agree that the opening of a novel asks a question (will the protagonist get the girl/save the world/find meaning in her life?), then the end of your novel ought to answer it. That is, after all, how you know you've finished telling your story.
However, there are lots of ways in which the question can be answered. It's perfectly acceptable to answer with a 'maybe', although do be prepared for some readers to hate your open-ended finale.
There are conventions associated with different genres to consider, too. For example, in crime fiction it's expected that the mystery will be solved (and, usually, justice is served). In a romantic tale, it's expected that the star-crossed lovers will end up together, with a happy ever after either described or implied.
Of course, you're free to turn these conventions on their heads, subverting reader expectation. However, if this is your plan, you might want to hint to your reader that you are in the genre-defying game before you finish a classic murder mystery without revealing the killer.
Essentially, reader expectation is all-important. If you're writing a literary work in which there are multiple viewpoints, an unreliable narrator or two, and several footnotes, it won't surprise your reader when your ending is ambiguous or even confusing.
There is a world of difference between an intriguing and emotionally-resonant, ambiguous ending, and one which inspires a disgruntled snort. For example, at the end of the film Inception, there was a visual motif (a spinning top) which called into question the reality of the final events. Of course, the entire film was about dream lives, reality and perception, so it was both appropriate and expected. It finished the film with an added air of mystery that honoured the preceding two hours of screen time. If, however, at the end of Die Hard, Bruce Willis had woken up in his bed wondering if it had all been a vivid dream, it would've been ridiculous, irritating and juvenile.
In structural terms, it's a good idea to finish your subplots before your main plot, although if these are closely linked, they may be resolved simultaneously. It's also not necessary to tie up every loose end, or to explain exactly what happens to every character. This is very much a 'gut feeling' thing, I think. If things are 'too neat', they can feel contrived and over-explained, but you also don't want to leave too many threads dangling.
Finally, the last line of your book should (hopefully) linger in your reader's mind. Make it a good one.