We have the feature to move active post back to the top of the forum when it gets a reply.  But, we tried it when we were testing this software and found it to be confusing.  Post were jumping all over the place and you did not know where to look for them at.  Several of our testers did not like it, so we turned it off.

You might want to look at using the email notification system.  You can subscribe to a thread and you will get an email every time there is a new reply to the thread.  You have to have a valid email address in your profile (enter it on the control panel) for this to work.

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