Sometimes most (or all) of what I want to say about an item is relevant only to me—guesswork about whether I'd seen it before, for example.
Right now, the only option is to create a regular old public comment, which might be entirely personal speculation. Ideally, I wouldn't force others to wade through my own housekeeping notes when looking at the comments for a show/film.
Just now, I've made it to where "Kickball 2: The Kickening" actually aired during Happy Endings, and left myself public comments in a few places to explain the weird episode order around that part of season 3. Some may be useful to future viewers, as they too run into Netflix/Hulu showing a different order than Trakt, but there's definitely some stuff that's only relevant to me (like analyzing my own history to deduce whether any plays have been lost to database reshuffling).
MyAnimeList has spoiled me with the "Private notes" field it offers for every entry (in that case, shows get it on the season/"cour" level). I just wish Trakt had either a feature like that, or a way to mark one's own comments as "not of public interest" so they can be hidden from the main comment views for other users and don't risk being flagged as off-topic.
+1 - wanted to make a note on a show to remind me who told me to watch it.
on the API side this is already implemented. If you make the api call to get items on a personal list, you will find a "notes" attribute, which you can also set via an API call. However, this notes attribute does not show up on the trakt website, only when you make a call to the api.