As others have alluded, many iconic books also sport iconic or classic covers. The distinction definitely becomes somewhat murky. Take Action 1 for instance: does anybody really think that is not a classic cover?? Of course it is.
Batman 1 - simple and classic. Silver age Green Lantern 1 - not so much. Looking through the annals, many key books also sport classic covers, but many of them do not get noted as such.
In my mind, an operational definition might be summed up thusly: a cover that makes a book dramatically stand out against others near it in a run, or an era, which is often desired more for its cover alone by collectors who otherwise might not collect that run or title.
ASM 151 is a sound bronze age example. I'd argue it's the cover that drives the demand more on a book like Hulk 340 than the interior, especially now some thirty years later.