For those of us who have sunk a great many hours into this game, there is a lot of new content to take all our time. Afterbirth is being described as "a mega DLC" that will add a long list of new things.
Expect to have 100+ hours of new gameplay content including Greed Mode, daily runs, 1000+ new rooms, and a plethora of other new things. You can see the full and very long list of new content over at the official website. October 30 is the day and $9.99 US is the cost. There will be pre-order and discount info soon, so hold on for that.