It would be nice if there was an option to compare two users' watch lists. The result would be a filtered list with all the dramas/movies that the one user has on there watch list but the other one has not.
This would be useful for recommending dramas to other users, as you could quickly identify all titles on your watch list that the other user might not have seen yet.
To accommodate for the different categories a color scheme might even be useful. E.g: green = "not on any of the other user's lists", yellow = "not in the same category (eg. on your completed-list but the other user has it on their plan-to-watch-list)", red = "the other user has dropped this title".
Another possible addition/function could be a comparison of ratings for shows that both users have on their respective watch lists.
