HalfWikisExamples might be:
Wishlists
You want to be able to add things to your wishlist, but probably not to have other people add them. You want anyone to be able to remove them, but possibly with some kind of reversion if the wished-for thing doesn't appear in a reasonable time.
Photo Albums
There are lots of uncaptioned photo albums out there on the web (because putting up piles of pictures is easy and writing captions is hard). One fix to this would be to let any random person add captions, but only that - they couldn't put in their own photos for instance.