Is it similar plot elements?  Technical details, such as what lighting filter a director chooses?  Is it a common character archetype, such as the “anti-hero” or “mad scientist?”  The emotions a movie evokes?  Length?  Simply genre?  Actors, writers, directors?  All of the above?

Or does it depend on the individual user?

It’s tough.  Movies are so darned multifaceted, that making comparisons between them is quite the task.  Competitors for the Netflix Prize have utilized users to determine what movies are similar; if person A and person B like movie C, and person A also likes movie D, then perhaps person B might like it, too.  Those movies are sufficiently ’similar’ to each other, based off of inferred preference.  Now, a great deal of sophistication has emerged from that simple model, taking other movie features into account.  We like to think that we’ve got a wealth of data to make some well-informed comparisons between movies.  And not just on a few of a movie’s overall features – on ANY feature worth noting.  So, if you like this movie’s cinematography, we’ve got a list of movies that you might enjoy.  Or story.  Or actors.  Or character types.  Or budget.  Or, …

Just food for thought.  Perhaps a brief sneak peak at what’s to come.