On David Bordwell's blog, he distinguishes between cinephiles (literally, those who love film) and cinemaniacs (people who tend to obsess over specific films), and although I think his "games cinephiles play" section is complete crap, I do like how he defines cinephilia.
The real crux, I think, is this. The cinephile loves the idea of film.
That means loving not only its accomplishments but its potential, its promise and prospects. It’s as if individual films, delectable and overpowering as they can be, are but glimpses of something far grander. That distant horizon, impossible to describe fully, is Cinema, and it is this art form, or medium, that is the ultimate object of devotion. In the darkening auditorium there ignites the hope of another view of that mysterious realm. The pious will call Cinema a holy place, the secular will see it as the treasure-house of an artform still capable of great things. The promised land of cinema, as experimentalists of the 1920s called it: that, mystical as it sounds, is my sense of what the cinephile yearns for.
This separates the cinephile from the lover of novels or classical music. They love their art, I suspect, because of its great accomplishments. Who with literary or musical taste would embrace the subpar novel or the apprentice toccata? But cinephiles will watch damn near anything looking for a moment’s worth of magic. Perhaps this puts cinephiles closer to theatre buffs.
That’s also why I think that the cinephile finds the desert-island question so hard to answer. What movies would I want to live with for the rest of my life? All of them, especially the ones I haven’t yet seen.