Last modified: 2014-02-12 18:48:37 UTC
What it says on the tin; the watchlist button is gone, at least from monobook and vector.
Ie8bbd0a40421d7d279fb896ebaa15a7de8cbd38a fixed this, it just needs a backport. And to be clear, you can (un)watch pages, just the tab is gone.
Note: The tab/star on the associated page, still works fine for un/watching. So nothing complicated is required for editors who want to un/watch Flow Boards in the meantime.
This is fixed in wmf13 which goes to wikipedia's tomorrow.
Taking Erik's comment as not needing backport.