Hey,
I'm a new user, so sorry if I missed something (and if it's there, you might want to consider making it more visible).
From what I understand, there is supposed to be a "mark all watched"-feature, but I just can't find it. http://www.tvmaze.com/threads/15/mark-all-watched
Here's what I see. It doesn't look anything like what's described on that thread.
