Talking about subjectivity...
I think it's just about the game being well known and good playable, in fact.
But personally I think a classic needs a certain atmosphere, to create a certain curiosity to go on with the game, or it needs a huge addictive factor. Let's say a "what happens if I do this" factor :-)
But these factors do not make a classic since if no one knows the game, or experience the game the way I do, it won't often be called a classic.
And there are countless of examles proving this.
