My issue with Improfanfic was the tendency of the various authors to forget certain significant aspects of various characters over the course of 45 chapters. Really, I think that comes from simply *having* 45 chapters, more than anything else - that and the fact that it's not a single author, so you get a sort of telephone effect, where author C is misremembering Author B's representation of author A's character. That, and the fact that it always seemd to end with either "We have to figure out a way to end this monstrosity *somehow*." or just petering out from lack of interest. I'd say that *part* of what a plot structure would do is give people a place to write towards, so that there was actual progress made towards a conclusion, and the stories eventually end.
Mind you, some of the ride along the way got pretty cool, but...
Mind you, some of the ride along the way got pretty cool, but...