When people list greatest albums, I expect popularity and impact to weight in heaviest.
Classic is not the same in my mind.
I LOVE albums like Firing Squad (since we just discussed MOP) and POSs Never Better (to mention a recent album) but I'm not calling those classics.
The term classic either has to not be used or used a lot less freely.
For example, if you go to amazon where many people review albums, you can better gauge different aspects.
An album like Illmatic will have like 99% 5 stars. That album is pretty cemented with classic status.
Look at an album like Get Rich and you will see a much bigger divide in the reviews.
And please don't harp on the amazon example and miss my point, which is that while popularity, success and impact play a part, when discussing classics, quality and critical and public acclaim also matter.