13: What makes a successful campaign?
The players. Probably the GM. Luck.
Okay, one element you can have some control over - what does and doesn’t go into the campaign. This can include tailoring to the group, who is at the table, but also in-campaign elements like the breadth of the setting, its Weird Level and so on. One of the reasons The Watch House lasted seven seasons (as did Buffy, and five for Angel) is that we could throw crazy high-concept adventures in with little or no warning or excuse. See above re Captain Rugged.
But it was ultimately the players and GM having fun at the table that kept them coming back.
(I’ll take “stating the obvious” for ten points.)
Part of that was the design of the game not to be high-risk in a way that can be stressful to play, with death and serious injury off the table unless the player in question found it interesting - so players could kick back, have their characters make jokes and be awesome in the face of horror, and some could bring the authorial distance and Whedon pain while others elected not to.