We've talked for years about a simple /feed/json/ endpoint. It usually gets bogged down immediately by everyone realizing we don't know how we should expose that data. There isn't a spec or anything like that.
http://rssjs.org/ was a proposal by Dave Winer last year to create a JSON-based RSS pseudo-standard. Essentially: Let's use exactly what RSS 2.0 exposes, in the way that it exposes it, but in JSON form.
I'm sure someone will mention rmccue & co's JSON API, but they are not mutually exclusive in the slightest. It would be super cool if every WordPress site had a consumable JSON feed.
I think /feed/rssjs/ might make more sense than /feed/json/. It hasn't taken off as a standard, so maybe not. (Or, we could be the ones to drive its adoption.)