Ability to suggest similar items and allow to vote for each suggestion/recommended item.
Added on May 8 15:01
First of all, thanks for your interesting comment :) I didn't know about Flixster.com... but it's an excellent example of what I'm wishing for :)
I think a special section for "similar movies" (for example) would be a lot better than lists:
1) more specific, accurate and selective (there are too many generic list like "my favorite movies", "best movies of 1999", "best movies of [director or actor name]", and you won't find what you're really looking for...);
2) visually better and immediate (a thumbnail-link of the movie poster for each suggestion);
3) a list of suggestion sorted by users votes.
I know that there are lots of criteria to choose from, but they're all good IMHO: similar story, plot, characters, locations, events, cast & crew, style, look, subject, themes, topics, mood or feel... But there will be a selection process by users votes (see point 3).
I think we're in agreement--I'm just trying to combine the best of both worlds. Flixster's system is good--it just could use some additions like the 3 points I mentioned and the cover thumbnails and sorting you mentioned. Sorting could be done by the number of Thumbs Up votes, Thumbs Down votes, or the difference between the two. 123 up and 2 down would have a difference of 121, which would rank much higher than a movie with 130 up and 100 down, with its difference of 30, even though the latter had more thumbs-up votes.
What I would say about the 'too many generic lists' problem is:
1. vote for this suggestion
2. "Best Movies of [director or actor name]" are just the kinds of lists I'm looking for and create myself because if I really like a movie, its usually mainly because of the actors, and the easiest way to find something else I'd like (even if its not related thematically to the current movie) is to look at the rest of the movies by that actor. Sure, you can just click on the actor's name on any listal page's Starring section and see a list of all their movies, but if a user has taken the trouble to make a list for that actor, you know they're a fan, and that gives their rating of each of that actor's movies more weight. If they rate a movie very low despite a favorite actor's presence in it, you're probably not going to like it either. On such lists, you can also see whether the actor's role in each movie is just a brief cameo probably not worth bothering with.
I would use this only if it was integrated with the compatibility system.
EDIT: Not compatibility instead Only if the voting can have a thumbs down option.
Make more community lists. This is hybrid of http://www.listal.com/suggestion/300