Great question. Personally, I think to be considered a "classic" in my book, a movie has to meet a couple of key tests:
1) Great dialogue - special effects will look outdated at some point, and action is just eye candy, where great dialogue and the relationship between characters stays with you, as do the lines and quotes that struck you throughout the movie
2) Stands the test of time - is popular with the generation after the one that first watched it, and the one after that. Movies that are believable, well written and acted and meet criteria #1 will be popular for decades and with different age ranges.
3) Offer some kind of revealing or profound commentary on the subject of the film or the time frame in which it was made, so that the movie becomes more than just a movie, but a reflection of society as well.
There's probably more, but this is the first time I've really thought about it. Interested in what others think...