As a mother, I know a lot about transitions. But knowing and mastering are two completely different things. Most kids don't like transition and they really don't like quick, unexpected changes. You can't move too quickly from play to work or homework to activities. Rushing them only makes the situation more volatile. But it's hard to space things out in a way that everyone flows gracefully from one thing to another.
I feel the same way when I'm reading. I don't like an abrupt and unexplained change in story, focus, or voice without any explanation.
A good book should be an interesting journey down a scenic, winding path--not a straight drop off a cliff. Some tense and surprising story changes can add to drama, but it has to be expertly done to not be jarring. I don't mind a maze, as long as I'm never left so frustrated that giving up seems easier than finishing.
Abrupt story changes leave the reader scratching his/her head. An author should give the reader as few excuses as possible to pull away from the story. If a reader's mind starts to wander, they disengage. Almost every 5-star review talks about not being able to put a book down. These types of books have seamless transitions.
Flashbacks and changing narrators make for tricky transitions. When it's done well, the story threads weave together into a beautiful tapestry. When it's not, it's a first time knitter's three-armed sweater.
How much stumbling can you take before you put a book down? What story elements do you feel are the trickiest transitions?