Regarding your point #3: how would "scrub" work with interconnected user data? Comment threads like this one are a good example; my comment is dependent on yours for context. Should you be able to delete your data and leave my comment dangling? If so, should your deletion cascade to my related data? Shouldn't I have a say in that?
Certainly that's a grey area, but I don't see why such posts couldn't just replace the user with "Unknown User" and leave the actual data there. Definitely there'd be a problem with conversations when some posts disappear. But I should still be able to detach myself from those things.
