Add groups/clubs that users can join (and tag members when watching films together, etc)
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.
Love this idea. Less serious suggestion in next comment ...
Matt Powell commented
I'm using tags to do something similar (e.g., tagging a film as "with:sophie"), which has dual benefits: it works for people who aren't (yet?) users, and it lets me see all the movies Sophie and I have watched together in a list. There's a possible problem if I know more than one Sophie, but so far this has been a pretty easy method of keeping track of which of my friends I need to spend more time with. Still, it'd be nice to have something official: an auto-complete list of people you follow, perhaps augmented with Facebook or Twitter contacts if your account has access to that data. You could even use this as an upselling mechanism: link non-users to an "invite to Letterboxd" thing?
Andrew James commented
Technically you can already do this. Just add an href tag and link to their profile. I realize not everyone knows how to do this - though you should if you're on the internet in 2012!!