I think "recommendation" will be a killer feature of the site, even if it doesn't approach the philosophy behind letterboxd (being a social site).
Moreover now that you are at the beta stage, meaning fewer users, less probability of "friends", the recommendation of a movie based on my most liked movies or the highest rating, it'll be a great way of discovering new movies.
I know that technically it will very challenging or maybe you have even think about it.

I'd like the option if keeping my account private, like how Twitter handles private accounts.

I imagine some kind of inbox or notification system where you can receive suggests from your friends. The film menu you see on mouse over can get an extra option "Suggest to friend..."

Allow a list being 'open' which then users can follow a list and add films to it. This will result in much more list activity and complete film lists.

Take for example http://letterboxd.com/pablos/list/special-powers/. I can easily add 10 more movies without any effort to make the list more complete.

Optional: to demotivate abuse, submitted films must be allowed by the list owner before being added.

The ability to list one's favorite films is great, especially when sharing preferences with friends; the opportunity to list favorite actors would be just as convenient. Adding a "favorite actors" category to a users profile page would be a fairly simple change that could really add a lot of personality to each account.

I often find that I want to list things other than films. It would be great if I could create a list of, for example, the Best Directors. Or list my Favourite Japanese Actresses. Or make a list of popular Animation Studios.

As with other "logging" sites badges would be nice. These badges add a bit of fun and competition to the movie watching experience. These could be badges about the content (Having watched all movies by Hitchcock, Having Watched Every Start Trek Movie...) about the amount (You have watched more then 100 movies this month, You have watched more than 1000 movies in total) or about the topic (Sci-Fi badge for watching at least 25 sci-fi movies and such things). I think it would be nice.

It would be great to be able to make a list that I could share with friends but not have public to all my followers. Like YouTube's "unlisted" setting on videos. Not public, not private, but sharable through links? Say, a list of films we plan to watch together that I wouldn't want on my page?

Most reviews are and will be written in English. But once public accessible this site will (hopefully) grow, so will the number of reviews and diversity of languages used to write these reviews. On a movie detail I can see 3 popular reviews in 3 different languages for example.

Maybe an options where you can select in what language your review is written and an option (checkboxes) to select which languages you understand... and wan't to see.

I have a lot of private lists for organizing my opinions for films as well as information for my YouTube channel and Instagram, and I keep having to search through my list longer than I thought. I would really like to see folders be visible, so that folders are visible in the lists section, while lists that can stay out of folders can be visible normally in the same section. Please take this into consideration!

Would be great if we could cross two or more lists generating a new list (temporary or not) as a result.

A greater possibility of films exploration.

I'm always dying to explore better giant lists like Palm d'or Nominees to see how many films an director has in it. Or another example to intersect this previous list with Criterion collection etc etc.

In addition to the Letterboxd's user average, have an average score of only your friends rating.

Perhaps there's a good reason for you NOT to include film ratings next to each film, but I feel like it's important enough information that it should be included at least somewhere. I have to exit the app to find out if the movie is rated R or not... a bit strange?

Do we really need the Popular Users page? it just turns the site into a popularity contest.

I don't think you should be deleted user's comments when they delete/deactivate their account. When the comments are deleted/removed it makes all the comment threads illegible and all messed up because people are responding to a comment that no longer exists.

Long-time Filmtipset.se user, now migrated to Letterboxd. What I miss more than anything else is an advanced search option. I use this very frequently to set up different lists. For example, thrillers produced in France between 1960 and 1964, documentaries produced in Sweden between 1930 and 1945 or short films produced in the US between 2000 and 2015. You get my point.

When I'm on my diary, and I look through all the films I've watched this year, I wish that there was a way of hiding all films that I've rewatched this year. This would allow me to view all the films that I've watched for the first time this year. Shouldn't be too hard to implement, and would make all the difference. That way, people wouldn't have to make lists like this:
http://letterboxd.com/ptabro/list/dear-diary-jun-2013/

Hope to see this feature soon.