The more I thought about that comment I wrote, the more I thought I should revise it and post it here. Not because I'm Mr. Great Ideas or whatever, but because I wanted to hear what other people think.
Do you disagree with any of my points below? Think they need to be expanded? Am I missing something?
I learn best when I am told I am wrong.
Anyway, here is the (revised version of the) post I wrote in that thread.
* The main plot of each book starts and finishes in the same book. No cliffhanger endings.
* Subplots can span several books. If you want to write in cliffhangers for the character's romance, family drama, career hell, that's dandy.
* The series has a set, specific type of story it tells. It's a police procedural. It's a comic narrative about raising kids. It's an adventure into a magical land of talking animals. Decide and define early in the creation process.
* The protagonist is someone who has a solid story reason for having these adventures book after book. It's obvious that a police inspector is assigned murder cases. Superheroes seek out supervillains (and vice versa). Spies receive assignments. And so on.
* The supporting cast is useful to the protagonist (as exposition devices, supporting muscle, wise counsel or any number of other roles singly and in combination) and are colorful characters in their own right. They're also the main source of your subplot conflict.
* There's a wide variety of antagonists available. Sure, a homicide detective will chase murderers, but each should be unique in style and temperament. Ditto supervillains, alien invaders or world-conquering megalomaniacs. That doesn't mean you can't have recurring antagonists, but just more than one.
* There's a world to explore. Each police procedural looks into a new part of the world of cops. Each monster hunting book uncovers a new, secret part of the monster's world. A book about school follows the student into new extra-curricular activities or social cliques. There's always more to discover (and more research to do).
* The series--and the protagonist--changes over time while keeping that whatever-it-is that made the series appealing in the first place. (Doesn't that sound easy?)
* A casual reader could pick up any novel in the series and follow along the story as easily as if they were reading the first one.
So... what do I have wrong?