The series goes on and on
Don't get me wrong, I love opened ended series. But when the books are a tightly connected, we are at book 14 and the author happily announce book 16-18 and states it isn't the end? I walk away.
I get having a long series, but when I have to re-read the books to remember what happend it isn't fun anymore. A part of it is also what May wrote about in her post at Smexybooks in May : Keeping your readers informed. It is one thing to say that ” I plan the series to be 13-16 books long, and then I'll write stand alones.” and one thing to say ” I plan to write in this world forever.” The first makes me keep reading. The second? Makes me walk away.
There are no series arc
Even if the series are focused on just one character, I want a series arc. I want that an event in book one sets up book 2, and book 2 in turn sets up book 3 and so on. If there is no series arc all the books start to blur, and feel the same around book 4.
The characters doesn't change
This is the most important for me. The characters doesn't change. They do the same mistakes again, and again. And it just never occurs to them that their actions affect the world around them.
The plot starts to feel formulaic
For me it comes a moment when I read a book, and I realises that it could have been any of the previous books by the author. The books might still be good, but the series isn't on my autobuy list anymore.