I would really appreciate the ability to mark an entire season (or even an entire show) as "Watched" at once. Also in the same vein, it would be nice if one could click the "Watched" button a second time to revert that episode, season or show as "Watched". In the case of an entire show (and only in that case) would I confirm the user's selection with an "Are you sure you want to set every episode to watched?" and in all cases I would provide the user with an "Undo" link after confirming the Ajax response with a DOM-inserted and user-friendly message.

Hope that helps. Thanks for reading.