I think that the biggest problem is that people don't know for what to use things like that. Here at HN population is smart and they will know what and how. But if you like to catch broader developer population I think it will be great to provide separate APIs for things like sentiment analysis, comment spam analysis, images or any other concrete problem solution. If you have a platform, why not to simplify all and solve someones pain.

Ofcourse, general API will also have an audience, but on this way you will separate yourself from players like Google.