Okay, a change in Flow notifications went out with today's release. As of now on Mediawiki.org, subscribing to a Flow board will only give you a notification that a new topic has been created; you won't be automatically subscribed to new discussions on that board. This will help to cut down on the repetitive notifications that people were getting.
We're also going to try out bundling "created a new topic" notifications, when there are multiple new topics created on a board that you're watching. Rather than going directly to the topic page, the bundled "new topic" notifications will take you to the board, sorted by newly-created topics. The plus, obviously, is that you get fewer notifications; the minus is that you may need to scroll down on the board to make sure you don't miss something.
We'll also work on bundling email notifications, and giving the emails a more helpful subject line.