The thing is, people need to realize that instant classics exist, but they are extremely, extremely rare. Most classic albums take a while to be determined classics. And anyone may speculate what could be a classic in the future.
Also:
- innovation (musically or lyrically or even techniques used to increase acclaim and listeners without selling out)
- influence (being influential does not make your album classic, and not being influential doesn't mean it isn't a classic, it's just that a lot of classic material has been so)
- critical acclaim (99% of classic albums are critically acclaimed, very rarely does a classic have bad reception from critics)
- albums with classic songs don't make the album classic. classic albums don't need to have a classic song.
- possible influence or marking of a huge era in pop culture (or, huge for any particular sub culture)