Agreed. It really comes down to what is enough to satisfy most people. Exporting subscriptions is an easy implementation. Saved/favorited posts, slightly less easy but very achievable. Each of these could be safely done as a user initiated export/import.

Once you start getting into any type of ownership type work, votes, comments, etc. then it's starts getting hairy due to integrity concerns. How do we trust that this activity actually belongs to the person claiming it.