Child abuse material on the blockchain
Say someone uploaded a child abuse picture onto the Steem blockchain. Being a blockchain, it would be close to impossible to delete it. So there it was, that image that would haunt the child for the rest of it's life.
The same goes for revenge porn, or any other type of content posted without consent.
Censorship may be a good thing
This type of case if a major concern of mine when it come to blockchain-based social media platforms. On centralized platforms such as Facebook, there is at least a small chance of such content being taken down and practically taken off the internet for good. With the Steem blockchain, you can't.
Jailtime for miners
With a child abuse picture on the Steem blockchain, a copy of that picture will find it's way onto all the Steem miners. What will they do when the FBI comes knocking on their door, only to find child abuse material on their mining rig?