Why is for example Back to the Future quotable, but Avatar isn't
shareQuality of line and application to every day usage.
For example "Who you gonna call?" can be applied easily on a day to day basis.
Also the delivery of the line in a film and humour quality helps.
Writing.
Delivery.
Because some are more memorable and some just aren't. Back to the Future is very reliant on its dialogue to tell the story, whereas Avatar is a movie that captures you for its imagery, not for its dialogue - it could be a silent movie. Also think it has a lot to do with how short and sweet the dialogue is and how it can be applied in your everyday life, whether as a common phrase or a joke.
shareWell written and catchy dialogue.
shareHaha, what a story, MovieElephant!
Anyway, how is your sex life?
A lot is in the quality of writing and the quality of performances, but some of it is in the "uniqueness", too. Also, some movies burrow into the zeitgeist, others don't.
Let's consider Gangs of New York and There Will Be Blood.
They're both Daniel Day Lewis-starring period dramas. They're both by highly regarded filmmakers. Both have award-winning directors, casts, and production teams.
Why do we all know, "I DRINK YOUR MILKSHAKE!" but we don't know, "Amsterdam? Well, I'm New York."
The former line is histrionic, and delivered in an over-the-top manner. It's not bad; in fact, it's excellent. But it's just easier to yell it at friends and get a laugh.
The latter is a BRILLIANT piece of writing that advances story and character, references history, and sets up conflict. I can't get over this line. But the delivery is more subtle, and to "get" all the layers of it requires a bit of history trivia knowledge and appreciation for the personal conflict.
Result? We can quote There Will Be Blood, but not Gangs of New York.
So it's not just quality of line and delivery, or even profile of the film (I'd say both films did well at the box office and were well-received by film geeks like me, but not so much average bloke moviegoers), but a combination of all of the above plus that "X Factor" where the milkshake line just has more meme potential.