Given in a probably disorganized fashion, my thoughts on the subject (by the way, I don't mean to be challenging or offensive to anyone; I'm just listing my observations):
I have found that, when time is left for character development, few, if any, people actually make use of it or post at all. What usually happens, from what I've seen recently at least, is that one or two people post, a succession of short wandering posts follows, and the TS stagnates until someone starts it moving again.
To keep the TS moving and to try and keep people interested, I have, in some cases in recent TSs, made a conscious effort to focus fairly steadily on the plot (my observations of previous TSs indicated that such a course might restore activity and keep a TS alive). That is, I have found that one way to keep participation up is to focus on the plot.
It is possible that the above method is only a temporary solution (and might not be a solution at all). The remedy for low activity might change over time and perhaps the above approach only worked in one or two cases. If anyone has any suggestions on this topic, please feel free to share them.
It might be that people don't enjoy the stories or the characters very much. I think in older TSs that people just had fun whatever they were writing. In recent TSs, people just don't seem to have the interest in the characters to post interactions among them. Of course, there is also the fact that many people are wary of using other's characters, not being certain how the owner of the character will react to the treatment of his creation.
The idea that Pippin proposed some time back might be a solution. If everyone has new characters, perhaps they will be more interested.
Mack: What would make you more interested in joining the stories? If you got to kill everyone else's character in every story
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?
I don't think that a turn-based system would help, at least not for all TSs. It might work for a special kind of TS though.
There has been some discussion about the next TS after Terminated. I think that it could be extremely interesting and enjoyable for everyone, but there are still some details to work out about it.