this is going to be rife with generalizations so i'll apologize right from the get go.
seems to me the bulk of classic rock is a guitar-centric take on an old blues tune and cranked up to 11. (it worked for the first few led zeppelin discs)
the guitar has always been a predominant factor in classic rock also, there are a few exceptions (like elton john) but for the most part the guitar takes center stage.
i think what really makes a good classic rock tune stand the test of time is the lyrical content. when the lyrics are more abstract and relative to broader life experiences the song can continue to be relevant to the listener long after it's initial release. on the flip side songs with specific static lyrical content can be appealing for nostalgic purposes for those who want to relive the past.
take for example stairway to heaven, arguably THE classic rock tune.
there's a lady who's sure
all that glitters is gold
and she's buying a stairway to heaven
that leaves tons of room for interpretation, you can take it as a literal fairytale about a giant staircase leading into the heavens or interpret it as a fool's attempt at spiritual enlightenment through a focus on material goods or a multitude of other meanings, and that's just the first half of the first verse.
on the other hand a song like "let's go to the hop", kind of looses itself on modern listeners since most of them would have to ask their grandparents what 'the hop' is. it's definitely a oldie but not classic rock.
then there's the even more vague relationship between the social and cultural environment that was in place during the 40s and 50s that shaped the early lives of the individuals that would eventually become the classic rock musicians. especially through the cultural revolution of the 60s.
also, unlike today i'm willing to bet the fair share of CR musicians all knew how to read music, and most had studied an instrument in school. the VAST majority of rock musicians under 30 can't read music anymore (i'm one of them). it's not really anyone's fault, it's just that tablature makes learning how to play guitar or bass really simple, unfortunately it's creating a generation of illiterate musicians. i'm not saying one is more right than the other, just pointing out differences.