It seems normal to me that we would have the Mark All Read at the bottom of the page. So that it is right there, when you are done scrolling threads. Right now, you have to go back to the top to get to it. Obviously not a huge deal, in fact there is a button to get back to the top. But, it would seem more ergonomic, I think, to most readers to have the 'done' button at the bottom, where most of us find ourselves 'done'.