Craig, this is possible, see image below - top right corner of any discussion has button "Follow":

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:

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:

I hope this helps, cheers!