Since Facebook allowed users to subscribe to other users, I receive a notification when certain friends update his or her status. I've checked the subscription settings of these friends, and I don't see anything different than those of non-notification friends. Does anyone know how to turn this feature on/off per friend? 
Facebook friend status update notification
facebook;notifications
If they are on your Close Friends list then by default you will be notified whenever they post.  To turn that off, click on Close Friends in the left sidebar (if it is not there then click More next to Lists and then click Close Friends).  Then click on Notifications at the top and choose Off.  Of course you could also remove the person from your Close Friends list.Groups are the same way.  If you do not want to receive notifications for posts to the group, click on the group name, then Notifications at the top of the page and choose Off.