Series Theories

What elements make for a successful series? I'D SURE LIKE TO FIND OUT, cause ya know? Neither of my series actually got to BE serieses (what is the plural of series??) to completion.

<RING RING>
Hello?
<Ms. Burnard? Bitter calling. Line two.>
Right. Right.

Ehem. Pardon me. Just gonna have this Snickers.

Ah. Where were we?
Series. Perhaps we can elucidate the elusive nature of the beasts by examining those aspects which deterred us from reading through other authors' works.

1. Top-That-Itis: Dreadful disease that fells many a promising series by forcing an author to reach deeper and deeper into the well of vast improbability for conflict. Some theories suggest that this tragic disease is what leads to that horrific event known as "Jumping the Shark."
2. Character Intellect Disorder: Characters afflicted with Intellect Disorder are easy to identify. Their authors spend great effort telling, and possibly, showing you how smart a character is only to have the character spend the rest of the series acting in a manner utterly inconsistent with that intellect. To put not too fine a point upon it, the character acts as if he or she has been clubbed suddenly stupid. This disorder may or may not accompany symptoms of Top-That-Itis.
3. Murky Motivation Mania: Some theorists contend that Character Intellect Disorder is nothing more than Murky Motivation Mania in action. *I* disagree, but the hair we split is fine, indeed. Any character can be forced to act out of character if properly motivated to do so. IE: a gun held to my head might indeed induce me to cluck like a chicken when normally, I would not. Anyone claiming the contrary is a fowl and dirty prevaricator. You see what I did there, yes? Ehem. However. One assumes that the moment I've dispatched the individual holding the gun to my head, I would revert to my true, uptig...er...sober mien. Therein lies the difference between Character Intellect Disorder and Murky Motivation Mania. The disorder appears as if a light switch had been flipped: Smart/Not. Mania manifests when a character acts out of character for no apparent reason and then just as mysteriously, flips back to acting as he or she always had.
4. Canon Fodder: Canon: A rule or law enacted by a competent authority. Alas, what mind among us can comprehend what madness besets an author who contradicts the rules of his or her creation as set out in the series as it first appeared? None of us. Not a one. We can only shake our heads in sad wonder and hope that Disney will redeem the franchise.

Come, my fellow series theorists. Regale me with your theories for what makes a series unreadable and that which makes a series eminently readable. I'll share the Snickers...