From an unanswered stackexchange post: http://salesforce.stackexchange.com/q/55833/60
My org has communities set up for partner users, and in our case they're highly trusted partners that we want to have access to chatter on all the records they can see, at least by default.
The problem we've discovered is that every time you make a new chatter post it defaults to "My Company Only" and the user has to remember to specifically change it to "All with access". Despite how simple of an action this is it's so easy to forget that it's leading to some awkward situations - like mentioning people who can't see the post!
My proposed fix is simple: have a preference available in the user's chatter settings to remember the default value. Ideally it could be set per object (e.g. all on opps, but my company only on accounts) but even a single global setting would be quite useful.