I'd likely consider this the most spectacular and wonderful comic book cover I've ever seen. It's a product of its time for sure. This Neal Adams masterwork graced one of DC's glorious 100-Page Spectaculars, Bronze Age treasure troves of Golden, Atomic, and Silver Aged goodness. Here's a link to a key with all the heroes identified.
At the time of this cover, these kinds of poster shots were exceedingly rare. This cover itself hearkens back to this All-Star cover from the Golden Age. In the modern comics world, it takes thirty heroes to have brunch, so these assemblies are not that significant. But to see this many heroes on one cover was breathtaking at the time.
The cover art must've been lost or damaged, because when this comic book was reprinted in the 90's, Dick Giordano was contracted to "recreate" the image. I was going to point out some of the distinctions between the two versions but at this link there is a detailed comparison of the two covers and the minor differences.
I only own the reprint, and Giordano's cover is magnificent. But that original was something else when it hit the stands so many decades ago. That's a beacon to adventure if I ever saw it.