With my GCSE students' wiki, a turning point seems to have been an idea which I adapted from Mike Wesch of putting 40 words and phrases on the wiki and giving the students a period of time to write something for each one. This was so much better than just asking them to use their wiki pages as a blog. By focusing their activity on a specific concept, they began to edit and comment upon, as well as add to each other's work. Of course, this is also what happens on the H809 student forums to a great extent. I think the nub of the difference is that we rarely actually edit each other's text. perhaps, as adults, this feels too discourteous? The truth is though, that my teenage students did this in a sensitive and supportive manner - for example, "I like that idea for solving a problem and this is another method that I sometimes use".
Last year's H809 wiki started completely blank... And pretty much stayed that way. This year I seeded it with some deliberately haphazard initial content, and there's been much more activity, leading to a valuable resource in progress.
Could be a random variation, of course, as happens from cohort to cohort (much less forum discussion this year, for example, but about the same amount of blogging). However, as per Janshs' observations, the "leavings" have typically been polite additions rather than edits that might be seen as a rude slight on a peer's contribution.
Now if the wiki content is carried over to next year's presentation, will that cohort feel less nevous about editing what would be seen as "old" text? Or would the distinction be quickly lost?
Visible attribution to a particular cohort might help initially, but perhaps following the threads of changes might become increasingly tedious as edits grew, and so the nervousness might continue. Opening up the wiki to the world (probably a GOOD THING anyway, so long as it didn't inhibit those new to wikis from contributing) would decrease the chances of it being a peer that one's edits offend, but make it more likely that the edits would be seen by an original contributer.
A wiki that consists only of additions and not revisions would be missing the chance to improve readability when accuaracy, coverage and detail are improved. At the same time, losing the norm of respect for peers would also be undesirable. So does a wiki made by such a community need additional etiquette compared with, say, Wikipedia?
And if so, this wouldn't be the only community with such a tension between a desire to improve a shared resource and a desire not to offend. How might one delineate such communities? Hmm... lots to think about...