What always surprises me as lurk around different forums is the hate series seem to get from some individuals, particularly series that reach great success like Harry Potter. “The author is just milking the cash cow,” they will say, or “Don’t they have enough money.” What these individuals don’t understand is that writers don’t write series to increase their money intake — at least not in most cases. Writers write series because they have a story they want to tell and it is just not physically possible to tell it in one book.
I will admit in some cases series are expanded for profitability. The Twilight Series (I refuse to call it a saga because it just is not one) is a good example as it was expanded at the request of the publisher, and it shows in the later two books where nothing happens for prolonged periods of time because Stephenie Meyer had only originally planned for one book. And that is one of the challenges with series: They need to be planned out thoroughly. Lack of proper planning is perhaps one of the big reasons people roll their eyes at series.
Without proper planning, you can have subplots that don’t go anywhere or are forgotten over the course of a long series, character whiplash, and a hoard of other problems, which always glare off the page to discerning readers. Writers must always consider the end point when writing a series; properly plan/outline the novels; write out character changes/growth/or planned back slides and the reasoning behind them; write down subplots and how they are resolved; and more.
While J.K. Rowling stated she had planned out the entirety of the Harry Potter series, I seriously doubt it. Why do I say this? The sudden inclusion of the Deathly Hallows in book seven. There should have been lead up to these magical items prior to book six — and no the inclusion of the cloak of invisibility does not cut it because the concept of the Deathly Hallows was not mentioned beside it. This could have been corrected if the items were referenced in one of the earlier books; perhaps, Hermione could have noted them from one of her readings, etc. If this had been done, readers could have had an “AH!” moment, rather than be left to scratch their heads and wonder why Rowling pulled these items out of nowhere.
The other plus to properly planning out a series is you have a rough idea of how many books you need to tell your story, and it should help prevent you from having a book (or books) in the series that does nothing — that does not push the plot through to the end. This is a real danger when writing series and will only serve to frustrate readers who will grumble. It will also help prevent wandering series that might outlive you, e.g., The Wheel of Time series.
No doubt, even if a writer takes the time and follows through with their plans, there will still be gripers and that is something that will never change. Even so, writers should not shy away from writing series; after all, the most important thing is for the writer to get out their story with perfect execution, so the plot bunnies don’t drive them crazy.