Huge +1 for long term backwards compatibility.
I can't quite see what problems this is trying to solve?
Reading through https://www.mediawiki.org/wiki/API/API_rewrite/Kickoff_meeting I don't see any argument for why we are moving to replace the current API. It reads more like "oh, this'll be cool"!
That said, I'm all for making Mediawiki more accessible to "apps"/mobile/whatever. In particular I'm thinking of o-auth (or similar), which is long overdue. Something like that would be very helpful for toolserver tools.
I'm just a bit confused about why we need a new api. What are we trying to fix? What are the goals? And most importantly, we need a clear idea of where the current API fails.
-- Chris