there should be user control over who can see what you post
I think this is basically impossible.
Only things that aren't required by the blockchain to work are possible to encrypt.
For example: You can't encrypt votes, the blockchain need to know who and for what you voted for, so that the rewards can be calculated correctly.
Another example: You can encrypt the memo text, as the transaction is only between 2 people (therefore you can use the public key of the other party to encrypt it) and it isn't needed for anything else.
About posts:
Well you could encrypt a posts content, but if you want to do that you could just as well use any other service out there that is more for private texting. Way more efficient and you don't have a problem once the encryption on steem is broken (or anybody changes private keys).