you can use the new errorformat, errorlang, errorsuselocal paramers to get localized, parsed errors; if you don't, most API modules will behave the same as before. Exceptions:
for query modules the error codes won't be prefixed anymore the way parameter names are;
for a few modules (see announcement) which put errors in some non-default location in the response data, that location might change and/or the contents might change from a single error to an array of errors.
Working on new filter UI for Special:Recentchanges; a bunch of backend patches for this landed in MW core recently, more will land soon. Will be behind a hidden preference for the time being.
Converted Flow and Echo to extension.json. This has caused a few minor issues for Flow and may cause some for Echo as well. Also, if you have open patches, they'll probably conflict now.
Going to convert PageTriage next; and FlaggedRevs after we clear out the CR backlog a bit there
Blocked: Tim asked ops for someone to assist him with puppetizing HTML5Depurate for a pilot deployment to the production cluster for our Q2 goal. No response yet.
See "[Ops] Need Html5Depurate playmate" mail from Tim dated 11/24
Parsoid-native <gallery> rendering + a bunch of other changes going out today (Most interesting for MCS: heading ids now assigned that match core output).
I still can not join the Hangout when not being a WMF employee. I know multiple people already looked into this. Please forgive me when I'm not trying any more. I still find the information given in this Etherpad very helpful, and love sharing relevant bits via a WMDE mailing list.
