Do you like your learning curves steep or shallow? I like a slightly above-middling curve. Pretty much every story has a learning curve, but the degree of curve and time taken before the curve flattens varies greatly from story to story. Let me break it down a little further, in case you're confused (I almost confused myself).
As you read, you're constantly learning new things about the story you're reading. They can be mundane, like the names of the characters or the color of the kidnapper's car. Or they can be fantastical, like the gravitational force on a fictional planet or how many heads a Ingoosariofus has. Either way you're learning. To avoid confusing readers, writers need to gain skill in developing reasonable learning curves. In short, a learning curve is a measurement of the number of things learned by readers over a duration. It isn't a one-to-one ration, however; fantastical elements steepen the learning curve far more than mundane.
In a literary story, the learning curve tends to be very shallow (assuming the reader recognizes the real-world setting). Most things to be learned are small and mundane. Fantasy and sci-fi stories typically have the steepest curve. If the tome has a prologue, some readers will put down the book because they know they're about to be hit with hundreds of years' worth of history and an explanation of the magic system. (Or so they think. The most talented writers know better.) A mysteries' curve depends upon how smart the reader is. If they pick up on the clues, the curve appears to be steeper than if they read cluelessly (poor word choice, but I'll leave it because it's kind of funny). When Sherlock explains everything at the end, the learning curve actually surges upward, causing a shallow curve from the first two-thirds to turn into a hill in the final third. Mysteries are actually non-typical in having much learning curve at all in Act III (or whatever you'd like to call it).
In many stories the learning curve goes almost flat toward the middle. The reader should feel familiar in the world by that point, even if it seemed crazy and whimsical to begin with. That is, if it's done well. The curve has a little resurgence at the very end if there's a "character aha moment". In that case, it's perfectly fine when executed passably.
What kind of curves do you like reading (and/or writing) the most?