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.

Add a match button to every profile so you can check immediately if somebody has the same taste as you.

You can use the like and/or rating of a movie to produce a list of similar movies/similar profile or a similar %.

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

Seems like a natural addition. Website keeps growing. Why not expand to add television shows? Have them as a separate category so they don't mix in with film ratings?

Besides liking a list, I think it would be nice if you can follow a list. Some lists will change or grow over time and it would be nice if people get a notification in their activity of it when they choose to follow a list.

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.

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.

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..."

Enable users to create groups they can join, such as "Filmspotting fans," "365 Movies in 365 Days" or groups by location, etc. sort of like Facebook groups.

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.

When you watch a movie in Plex it marks it as watched here and at the end, it gives you an option to review/rate it which shows up here and that you can toggle on or off.

Just a thought, but not sure whether it's possible or not. Seems likely and it'd certainly urge me to permanently use the service.

Stunning work so far! Keep it up.

Pull our movie viewing history from Trakt into Letterboxd. Maybe also sync our ratings and/or reviews between the services...

For example, you could have
1. Citizen Kane
2. The Godfather
2. The Godfather Part II
3. Blade Runner

This shows that the two are considered to be equal on the list and doesn't mess up the numbering for the rest of the list.

In the 'films' section of your profile you can order your films by popularity, could it be possible to integrate this function into the 'watchlist'. Tis could be handy for days you have no inspiration on what to watch.?

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 very nice if you could have collaborative movie lists like Spotify does with their playlists. Or if there was some sort of group page feature where u and other people in the group could also make recommendations postings to each other or something similar.

I'd love for letterboxd to have an xbmc plugin (www.xbmc.org) which would allow to import your movie collection as well as collect seen/unseen data automatically. The option to rate a movie aftter watching it could be added too! Take a look at www.trakt.tv they have such a plugin and i think it would be awesome if letterboxd had something similar! Thanks
PS I`m typing this from my couch, in font of my win7/xbmc htpc powered home theater ;)