Default Post Model
Conclusion: posts default to checking the relationship between users
Are we going to be following a post model like twitter or tumblr (pretty much public, where except with private accounts any invisibility or privacy is more or less purely accidental) or like facebook (where your posts are more or less restricted to people you've already formed connections with).
The tagging features have very different implications if they're going to be used mostly for content filtering within your network of connections than if they're going to make your posts visible by using them for content discovery.
There are pros and cons to both methods, but the issues that have been being discussed have all kind of been assuming one method or the other but we should probably know where we stand on this.