Support notes for list entries (or comments for people)
The "Personal Recommendations" list already supports notes (up to 255 characters) to optionally state why one recommends an item (but AFAIK notes aren't supported by the website yet).
IMO it would be a good idea to extend this feature to all lists and maybe also increase/remove the 255 character limit.
This would be mainly useful for lists of people (actors/actresses, directors, writers, etc.) as it isn't possible to assign comments to them.
With an optional note one could state why that person is on a particular list (e.g. X is one of my favorite actresses for her performance in movie Y).
Alternatively this could be solved by allowing comments on people (different advantages and drawbacks but might be easier to implement (DB, website, and apps)).
I just noticed there's already a suggestion for watchlist notes: https://support.trakt.tv/forums/188762-general/suggestions/7317491-let-us-remind-ourselves-why-we-added-a-movie-to-ou
That seems like another reason to extend the notes feature to all lists.