A big problem from what I recall were legal ones. E.g., owners of a website did not fancy random comments graffitied over their content. I believe a few lawsuits were filed. Also spam problems arose. This caused the makers of such software to hobble functionality, making the resulting product less interesting and so these products were not successful.

This may be a good application for decentralized storage with free browser extensions that would circumvent much of the legal threads - although I imagine lawsuits could and would still be filed against Chrome, Firefox etc for enabling such extensions, even if they were free.