Pacing is critical to a good story. The writers' decisions not only on what happens, but when it happens and how quickly events transpire can determine whether your reader or viewer is going to make it to the end of the tale or give up in frustration.
An even pace throughout the whole story is rarely effective, unless you're writing a Slice of Life, where the steadiness and ambling nature of the pacing is an asset. In most other genres, though, that same steadiness kills any dramatic tension, so the writer will make decisions on when they speed up the action to further the plot, and when they slow down to give their audience a breather.
It can be tricky to get those choices right, however. Often, the audience will be faced with a glut of action (where they can't easily keep track of what's happening) or long stretches of time where it seems as though nothing's happening. The results are Pacing Problems, where the general feeling is that more sentences here and fewer there could have improved the whole book.
However, Pacing Problems are generally one of the more forgiveable issues a story can have. Very few of them will render a work automatically unwatchable or unreadable, unless the writer has really screwed up their timing.
Exposition is closely tied to pacing. The audience needs to know what's going on and why, but explaining all this can make them bored or confused. Show, Don't Tell might help get more exposition across while avoiding pacing problems, but if everything is shown through Flash Backs, that can be just as tiresome as hearing it all from Mr. Exposition.
Most people recommend at least getting through the beginning of a story before you give up on it, since perhaps the writer themselves was just getting into the swing of things...but a clumsy ending is much harder to stomach. Thus, Pacing Problems are split up into the points they occur in the timeline:
- Developing Doomed Characters: When the story spends too much time with something that isn't important.
- Intro Dump: Altogether too much information thrown at the reader at once in a big, ugly blob.
- Lost in Medias Res: The writer just can't wait to get started, so he throws the reader in at the deep end and expects them to figure things out on the way.
- Prolonged Prologue: When it takes too long just to get up to the first act.
- Slow-Paced Beginning: The start of the book, or just the prologue, is so slow and dense that the audience wonders if the story's started yet.
- Arc Fatigue: When there's just that one subplot, arc, or story that just. won't. end, while all you want to do is get it over with and get back to the real plot. Often a case involving a Romantic Plot Tumor (characters would rather develop a romantic subplot than help save the world or stop the villain).
- Exponential Plot Delay: When the real plot moves briskly at the beginning before slowing to a crawl.
- Filler: When whole chapters/episodes contribute absolutely nothing to the main plot and are only there to make up the word count/screen time. Happens most often in television series rather than films or books, particularly adaptations of ongoing print media series that must be produced at a slower rate than their television counterpart.
- Padding: This is filler on a smaller level, and usually happens to books, songs, films, and individual episodes.
- Plot Detour: When the characters, against all logic, ignore the main plot to pursue something unimportant.
- Cosmic Deadline: When the reader is hit with a glut of action (usually with a helping of Deus ex Machina) right at the end of the tale. Any dangling plot threads are solved here or totally forgotten about.
- Ending Fatigue: Something of an effect more than a cause of Pacing Problems, this is when the reader loses interest before the end out of boredom, plot incoherence, or just plain disinterest, and outright stops reading.