Not to mention that some standalone books can suffer horribly from the nothing-important-happens problem. (Tad Williams, I'm looking in your direction - and with the same gaze I use to nail people to walls, I might add.)
Another key warning sign is when you start seeing re-runs of character emotional arcs. C.f., oh, I dunno, a gazillion comic books, but also Series 5 of Babylon 5, wherein the writer/producer had had to advance a bunch of stuff to finish the major story arcs in Series 4 (because everybody thought the show was over), and then suddenly found themselves with the fifth year they'd wanted all along. This left two major characters (G'Kar and Londo Mollari) without nearly as much to do as they'd have otherwise had, so their interaction arc from the second half of Series 4 kind of got rerun in more detail in Series 5. It really detracted from that year. I rather suspect that Series 5 showed it the way it had been originally planned, but because we'd seen them cover this ground already, it just didn't work right.
(no subject)
Another key warning sign is when you start seeing re-runs of character emotional arcs. C.f., oh, I dunno, a gazillion comic books, but also Series 5 of Babylon 5, wherein the writer/producer had had to advance a bunch of stuff to finish the major story arcs in Series 4 (because everybody thought the show was over), and then suddenly found themselves with the fifth year they'd wanted all along. This left two major characters (G'Kar and Londo Mollari) without nearly as much to do as they'd have otherwise had, so their interaction arc from the second half of Series 4 kind of got rerun in more detail in Series 5. It really detracted from that year. I rather suspect that Series 5 showed it the way it had been originally planned, but because we'd seen them cover this ground already, it just didn't work right.