Love the buttons. Maybe you don't need them on the bottom as well as on the top. Just a thought, not a biggie. Also, would it make more sense for the Mark As Last Unread to be called Mark As Last Read? I tried it, it is helpful.
In edit: possible bug: When I read the last message on this thread (the one by NoMoDo) and then went back to Favorites, the counter hadn't reset. That's new I think.
In further edit: yep, I repeated this one on Zeev's thread. I went to Favorites and saw a new post on Turnips. I clicked on Turnips, not on New Unread, and got a new post. Then I returned to Favorites and saw 1 new unread post listed. Went back to the thread, but it was the same post.