Previously, you only received notifications for things concerning you. When so-and-so commented on a card or invited you to a board, a little red square with the number of unread notifications popped up in the header to let you know. All your updates are kept in a tidy list that you can always come back to.
The problem was that lots of people wanted to get updates about other cards on the board. Important things tended to get quickly buried in the activity feed. Occasionally people would assign themselves to other cards just to get updates, making it difficult to see who was actually working on what.
There was obviously a problem here. We wanted to keep you up to date, but didn't want to flood you with notifications about every card on every one of your boards. That would be too much.
You can now subscribe to any card. When you subscribe, you get a notification about anything that happens to that card: when somebody comments, when somebody gets added, and when it ceremoniously gets moved to the done list. To subscribe, just click the subscribe button on the back of a card. If you see the green checkmark, you'll get notifications.
You'll be automatically subscribed when you join a card, just like before, but you'll also have the option to unsubscribe and turn off notifications. You can also subscribe to cards on public boards to keep track of your favorite upcoming features, like, say, Templates and Copying on the Trello Development board. No need to check back or bookmark the board. When something happens, you'll know.
That's subscribe. We hope it keeps people more up to date and helps you keep track of the stuff you're interested in. It's live for everybody. Go try it out.
Another thing... In the interest of keeping people informed about all the latest and greatest stuff in Trello, we've added an in-app announcement system. When we launch a new feature (like subscribe), we'll send out an announcement. A little red mark over the 'info' button will show up, and you'll get a short blurb about it when you click. There's also a place for all recent announcements, in case you want a reminder.