Other than the writers were out of ideas, budget issues, writers leaving, and/or not knowing if you are renewed. What plotlines do you think failed?

Cute kids no longer being cute or tired of having to act cute (Diff'rent Strokes)
Couples getting married (I Dream of Jeannie)
Key cast members dying (Dennis the Menace)
Moving the main characters to another city (Gimme a Break!)
George and Susan's engagement (Seinfeld)

* Original writers being replaced, with the new writers not caring about previously-established continuity (The Facts of Life)
* Salary disputes causing cast members to depart
* On-set disputes causing cast members to quit
* Network executives, cast members, or writers having substance abuse issues
* Producers not seeing the value of certain characters

* Producers not seeing the value of certain characters

Good example would be, the entire cast of Family Matters minus Urkel, Carl and Laura.

Good example would be, the entire cast of Family Matters minus Urkel, Carl and Laura.

Absolutely. I still can't believe what a travesty that show became.