To give an example ... Here is what a retail partner is building with these services around specialized marketing: They have (or get) a user's twitter handle, feed the content authored by the user into the user modeling service and based on the characteristics, send appropriate coupons/offers (e.g. very adventurous or less so etc). Combining that with the machine translation & language identification services, they are able to broaden this to many more languages vs. just English.

The part that they are doing next is: with the community of their users, they want to check the potential resonance of their marketing messages to those users, which they do with the Message Resonance service. Based on that, they can pick the most likely to be resonant message before sending it to the user.

Was expecting people to talk about apps centered around delivering healthcare advice/information cheaper/more readily to people, turns out we're just going to keep finding ways to use tech to market ads to people.

The APIs can be used for all of the types of apps you mention -- there's nothing limited to, or even targeted to advertising. Here's an idea for an app: Imagine that you're a non-native English speaker, and you want to communicate with English-speaking friends on Facebook or Twitter. Language identification can detected your language, machine translation and translate it to English, concept expansion can automatically generate potential synonyms, message resonance can select the synonyms that best capture your message, and user modeling can tell you what kind of emotions your message is communicating. This technology can and will be used to bring people together, but ultimately it comes down to what types of apps people make.

create a persona that can have a conversation with people

see the movie "Her"

http://www.imdb.com/title/tt1798709/

how many people will fall in love with Watson?

Let's call it Emma Watson then.

I already do, and don't think I'm alone...