How important are chapter endings? Really, really, really, really important. As I work through my first round of revision on my YA, the round where I focus on plot, I pay particular attention to how I end each chapter. My goal is to make the reader want to -- scratch that -- need to -- turn the page.
Not every chapter ending needs a traditional cliffhanger (but having a few of those helps). But every chapter needs tension, and every chapter ending should somehow deepen the tension. A chapter could end with a character experiencing true love for the first time, but the reader knows from prior chapters that the love will be short-lived. Or a character could make a decision and accept that it will lead her into grave danger. If the reader is hooked, the page is turned.
So, as I write, and I look for ways to keep that forward momentum, I keep in mind the big "what if?" When I get to the end of the chapter, I ask, what if she slams the door in his face instead of inviting him in? What if he sucker punches him instead of shaking his hand? What if the dog drags a body part out of the woods (gruesome, I know) rather than hovering by his master's side?
I may not change the action of the scene, but by shaking up my initial impulse, I come up with chapter endings that ratchet up the tension. Sometimes these new directions lead me to completely rewrite the scene, or add another plot line, or introduce another character. That's all good. That's revision. And I love revision.