To me there is a substantial difference between what is popular and what is perceived to be 'good' with regards to movies. Popularity does not always equal quality (and, to be honest and little bit snobbish, I think they diverge more often than not). I'm going to address what makes a movie 'popular', as opposed to 'good'.
Thee main thing that a popular movie needs in this day and age is a sound marketing campaign behind it. No one is going to see a film they haven't heard of, and even if has many of the features I'll describe in more detail below, if no one has heard of the film, no one is going to see the film. As well as this, a good - and especially underground - marketing campaign can help smaller films with lower budgets claw their way into relevance.
Black Swan, for example, was never anticipated to become a popular movie due to its ballet theme and strong sexual element. However, an impressive trailer and a corresponding marketing campaign helped catapult the film into the zeitgeist and subsequently make over $300 million worldwide.
The next criteria to build a popular movie is to have an easily digestible 'high-concept' plot. A high-concept plot is one that's easily explainable in one or two sentences. Coming up with high-concept movies has been the job of numerous studio executives since the 1980s (and a little bit before then, if you want to get technical) when Jerry Bruckheimer and Don Simpson revolutionised the industry with
Top Gun. For the most part, people don't like complicated movies (or, at least, movies that appear complicated on the surface, like
The Tree of Life); the simpler the concept, the easier it is to understand, and the more likely it is that people will see it. That's not to say that complicated films cannot be popular, but they have a much greater struggle to find an audience, as they have to rely on a marketing campaign that tows the line between explaining the movie and not spoiling the movie.
If you haven't got a high-concept, your movie can still be popular if it has a fanbase already associated with it. Movies that come from popular books, TV shows, remakes, sequels, reboots etc. They all have a select fandom behind them, and make it easier to pitch a film to. Hell, even boardgames are getting in on the act! We've got
Battleship being released next year and
Monopoly already in development!
Selection of actors can sometimes help, but their role has become increasingly diminished in recent years. The Forbes List of Actors who add very little worth to their movies include some potentially surprising names such as Will Ferrell and Reese Witherspoon. Your Brangelinas and your Jennifer Anistons can still rake in the big money, but that's often because they play the same type of roles in the same type of films, allowing audiences to associate an actor with a type of film they like. As actors are more willing to diversify in the hopes of
Oscars peer approval, the subsequent star power has been diluted over the years as once-bankable stars are doing more and more indie-films without any of the pre-requisites mentioned above.
Finally, quality can help a movie become popular, but it's a very indirect process. Positive word of mouth is the best thing that can happen to a movie, and the biggest success story in this regard of this year would have to be
Bridesmaids. Box-office analysts were pessimistic on its prospects due to it featuring a large female cast (which has traditionally never done well with movie-goers, although there's been heavy debate about the sexism in analyses such as this), a first-time writer, and a bumper summer movie season. Yet word of mouth was ridiculously strong, and it became the little film that could over the summer of 2011. If people
really like your movie, they'll tell their friends and more people will come in the future. If they like it, they'll tell more friends and so on and so on. This is the best way to make a movie popular, as it lasts long after the marketing campaign has gone away and people have read the synopses on IMDb/Wikipedia. It's just becoming increasingly rare these days because of trailers and a greater awareness of what's around the corner in terms of movies.
Well, I've blabbed on way too long. Thanks for making such an interesting topic, O'Keafe. :D