(Which do you prefer?)There are two basic types of books out there - ones that are meant to be part of a series of inter-related stories, and ones that are meant to be the only one in that particular universe. When we, as writers, get an idea, we have to decide which of these paths we're going to travel down.
I have a preference for stories with a definitive beginning, middle, and end. Although I get into the characters I'm reading about, I prefer the story that's advancing. I'm not real big on getting into a character's universe and just following their lives as they plod through whatever big event happens to be going on. This is the biggest reason I've never been able to get into the James Bond movies - there's no real suspense as to whether or not old Jimbo is going to make it, because there'd be no story if he died.
Don't get me wrong, there are some great series out there. Harry Turtledove's saga about the way the United States would have been split had the Confederacy won the Civil War, or his telling of the way an alien invasion during World War II forever altered the way our world works, were awesome series. However, each had a place to start and a place to end. **SPOILER ALERT** In the How Few Remain series, the United States eventually reunited the country, even controlling Canada, and were going to put the pieces of a shattered nation back together. In the Worldwar series, mankind made it to the alien Homeworld and overtook them in military technology. I was satisfied with the end of each series, and it didn't leave me wondering if there was something else down the line.
Most of Stephen King's books, with the notable exception of the books that follow Roland Deschain in the Dark Tower series, are stand-alones. No one ventures back into Derry to see if Pennywise is still around, nor to they follow Ben Mears after he destroys Kurt Barlow. I can pick one of them up in peace, knowing that I won't have a lot of unanswered questions by the end.
Writers have to be careful about this. The worlds we create can be like warm blankets, comforting us when we return to them. However, if we're to keep readers satisfied and following us, we can't just create an everlasting series just for the sake of doing so. I believe the plot is the most important element, and our books must have a coherent story, not just some rambling tale about a guy as he makes breakfast in the morning before his daily bowel movement.
Of course, I might be in the minority. Which do you prefer? Do you want a set story, whether through a series or a stand-alone? Or do you like following a world with a character, unsure that anything will ever get fully resolved?