This looks interesting, as I use imdb mainly for lists, and they are too inconsistent to me - there's a watchlist, but no watched-list, for example. Or I haven't found it, because the ui is so inconsistent.
Does Letterboxd have the option to export your activity / lists?
Maybe Trakt would also be an alternative for you? They have a "watched" list as it's their main feature. Lots of media centers automatically submit what you watched to the site so it's great to keep track of things.
Here's what it looks like for a profile with some data:
on TMDb you have discussions, can import watchlist and ratings from IMDb lots you can create any list you want (watched but not rated as you mention for instance) plus of course it has discussions for each movie
