Sometimes, when I read a post in one of my threads and then want to think on it a bit before answering, I keep wanting to flag it so I can remember it has stuff in it I haven't responded to yet. Maybe it's just me and my organizational style, but I think it would be something useful. At least to me.
In the upper right corner beside the reply and notify buttons is a mark unread option. :)
They appear at the top and at the botton of the posts. However, if you started the thread yourself, then the one at the bottom is replaced by "ADD POLL".
In my humple opinion it would make more sense for the one at the top to be replaced instead of the one at the bottom, as you've just read towards the bottom and then think "I want to read this again at a later time." or at least be reminded at it at a later time, while the poll will appear at the top, so it makes sense to put that button where it'll appear.
Haibane: First, the Notifications aren't the same thing as Show Replies. Deleting all your posts from a certain thread DOES remove it from your Show Replies list, but yeah, it's frowned upon, so I'm not gonna repeat that.
Second, you can disable individual notificiations:
Go to the My Topics page.
Click on the "click here to go to the notifications section of your profile" link, which is on the top of that page.
Click the checkboxes right of the threads you no longer want to follow.
Click the Unsubscribe button.
(Clicking the checkbox at the top will flag ALL threads, and thus remove all of them.)