[chrispenycate]

As far as I can tell, this thread is mixing (with no particular organisation) a number of quite separate subjects.
Multivolume works (where a single story is too big to fit into one book, and minimum effort made to cover the joins. (Lord of the rings)
Planned series, where each book tells a story, but characters and environments carry over (Honor Harington)
Spinoff series, in an established universe, using different characters (sometimes not by the original author)(Saganami, with the precedent)
Developing series, where the author wrote the first book leaving enough loose ends that, when it sold, there could be later volumes (Dune)
Sequels, where an author is persuded to go on writing after a story (or series) has had its loose end tied up (Dragonriders of Pern)
Multiple stories told in a developing, growing universe, which may have common characters, but whose only necessary consistancy is that they not contradict each other (Known space)
Convenient universe stories, using a setting and occasionally characters to tell a number of different stories wthout building from scratch, frequently written by fans as well as the original author (Valdemar)
Fexible universe stories, where consistancy is not required, and an occasional contradiction is par for the course, which can generally be read in just about any order (Discworld)
Any of these (even the fan-written ones) can turn out good books; or fail to. It's not because a series was meticulously pepared from the start that it's going to maintain it's original quality. But the differentsituations have different pitfalls, and should not be judged by the same criteria.