Hi Cal,
To get reply notifications, you need to switch them on in your profile (click 'Profile' near the top of this page). You can also choose which topics you want replies to by scrolling to the bottom of the page, and on the left (just underneath the 'New Topic' and 'Post Reply' buttons), you will see 'Watch this topic for replies'. Click that, and you will get emails when someone posts. You can do that for any topic, not just ones you have posted in.

Hope that helps!