Well on Bungie.net if you read a post and lets say you make a comment and you would like to look at the thread again but you don't want to forget. So there's a button at the top that you click and it puts it in your Saved Threads page. When you want to look at it you go to your options and click Saved Threads and it displays all your saved threads. Check it out make an account on Bungie.net.
None.
Basically a light version of subscriptions.
Possible in the future.
None.