Sometimes when you are at a half way point in writing a book you feel like you have played all your cards when in fact there is much more to write. I sometimes get stressed and try to write things out but then get an idea and forget to work it into what I have already worked everything else out. I really need to sit down and work things out. I think writing a series is hard because you want to provide everything to your readers and carry that characters attitude they love on into the next book but at the same time have that character grow with their ongoing life. At the same time you are possibly adding new characters and finding ways to work them in and introduce them. At the same time this is happening not lose track of all your characters from your previous works. If they are there in the next book you need to not leave them out. You need to keep your story line together and not be running in so many directions that your reader gets lost to. You need to keep up and make sure you can work everything together and everyone has their place. So if you write a series just from my point of view its not as easy as you may think it is but it is fun. How much you enjoy it depends on you the writer.
How much your audience enjoys it depends on how much you put into it and keep things tight so not to confuse