Craig, this is possible, see image below - top right corner of any discussion has button "Follow":
![enter image description here](/c/portal/getImageAttachment?filename=2014-08-09_04-32-55.png&userId=113605)
If you click that even on a discussion that you did not participate it will appear in your profile under "Discussions" -> "Followed", which is a sort of bookmarking mechanism:
![enter image description here](/c/portal/getImageAttachment?filename=2014-08-09_04-36-23.png&userId=113605)
You will receive email notifications for "Followed" discussions. Also I would recommend not to use term "Flag" for this because it means a different thing and is also available on Community. To "Flag" means to sent private message to moderators about usually a troubled post that needs some attention - via clicking "Flag" button in the lower right corner:
![enter image description here](/c/portal/getImageAttachment?filename=2014-08-09_04-41-21.png&userId=113605)
I hope this helps, cheers!