There are some benefits to a sequence of novels. It could, for example, develop a following and a loyalty among readers. If a publisher contracts you for, say, a seven-book deal, you know you've got job security for seven books (unless, of course, you're suffering from writer's block; God help you). Nevertheless, the problems persist.
The first book in a series is the most difficult, especially for first time writers who cannot rely on name recognition to ensure a readership. In the first book does not catch the attention of the readers, then they won't want to pick up the second or the third of whatever number of installments there are in the rest of the series.
So, on the one hand, you have writers like J.K. Rowling and her debut novel Harry Potter and the Sorcerer's Stone (or the Philosopher's Stone, for you Brits). By and large, it's a stand-alone book, meaning that you can quit Harry Potter and not have to worry about any lose ends. Because of the warm reception the book received, Rowling went on to add another six books to the series. Without that positive first response, the publisher would have likely ended Harry Potter right there. And where would Daniel Radcliffe's career be then?
On the other hand, you can leave lose ends provided that the story is good enough to leave readers wanting more. Harry Turtledove is probably the best series writer I've ever read because of this. In the Balance, the first installment of his World War series, has a wide selection of characters from American infantry to Chinese revolutionaries to Jewish holocaust victims to alien conquerors. Such a large ensemble cast would up the odds of most readers finding at least a few characters to root for. He gives us a wonderful introduction to a 1940s Earth invaded by richly-developed aliens, has a number of characters endure harrowing experiences, and ends it with one of the characters on a search for his wife.
Speaking for myself, there were certain characters that I did not find particularly interesting, mainly the Soviet characters. On the other hand, I found most of the Americans engaging, and I kept wondering whether or not the holocaust characters would live or die as the Nazis fought the aliens. This brings me back to the point I just mentioned.
Another thing that Turtledove does very well is distill each installment of the series to a central event and links these to an arc that covers then entire work. That arc is nuclear weaponry. In In the Balance, humans steal nuclear materials from the aliens. In the second book, Tilting the Balance, the Soviets develop and use the first atomic bomb. The third book, Upsetting the Balance, has more nations armed with nukes and going to town on the aliens. Finally, the last book, Striking the Balance, has the aliens confronting this crisis and reaching a stalemate between us and them. I'm not saying this is the only thing that happens in the series, but it does help to streamline the whole. This is helpful for a writer to understand because it's very easy to get lost in the details of your writing and lose sight of what you series is about. Pick a storyline, any storyline, and use it as an organizing principle.
Oh, and, since I'm discussing Turtledove, always...leave...the reader...wanting...more! Turtledove goes all the way with this rule. At the end of the World War series, we know that there's more to come; from the first pages of the first book, we know that there are more aliens en route to Earth and Turtledove shows us what happens to these aliens in his follow-up Colonization series. Even Homeward Bound, the most recent of these novels, leaves me as a reader feeling that there's more to come. Whether or not Turtledove delivers is, of course, up to him.
I should emphasize, however, that Turtledove did have a bit of name recognition at this point. By the time In the Balance was published, he was already halfway through his Videssos novels, which began publication in 1987. While Turtledove is probably best known for his Southern Victory books, these did not come into print until the late-1990s.