It has to have the kind of appeal that doesn't wane with time. Upon release it usually has something fresh and innovative in terms of technical specs, or maybe it may just have some element of gameplay that is original or an implimentation that is novel.
I would say if it has all these things, it is a classic. Furthermore if it has two-player or multiplayer features it tends to stick in peoples' minds for longer...look at Worms. That is a true classic. I would suspect that many of the football games would also be classics, even though I don't go for them personally.
