Zad: Of course, there are those that will disagree with that assessment.
But, I assert that Def Leppard Hysteria is not a classic, period. Not merely less of a classic than insert album here.
And I recall you wrote that you would like to see MR's be the foremost metal review site on the web... a good step in that direction would be considering the many classics that are mentioned in "that other thread". Not bagging on the site or complaining, only trying to offer a little constructive criticism.
Holy Terror: Awaken the Guardian is definitely a classic. So is Seven Churches. Arguing which is more classic is futile. For death and thrash, Possessed's is arguably more classic. For power / prog, of course Fates Warning... but Seven Churches pre-dates Awaken the Guardian by a year, so, there's that.
I still would like to see some Thin Lizzy before either of those, though.
As for what is considered a classic and what isn't, I think the classic request thread is a good indication of that