I noticed that there is a "MARK" button between the "REPLY" button and the "NOTIFY" button

And if you'd like a preview, look here. 

Added new mark section (ToReply) -- so now you can mark threads in a new list to keep track of threads you want to reply to later.
Now you can mark threads as:
Bookmarked, Rated, Ignored, ToReply

if people have additional ideas for a new section of marks let me know.

now you can mark threads in a new list to keep track of threads you want to reply to later

Handy, thanks!

maybe ill add a feature where it automatically removes the item from your ToReply list if you reply after adding it (right now you have to remove it manually, like you do for all items).