8.5.1 was an OS that was more compatible with Classic Macs, but 8.6 was built for the newer breed of iMac/Yosemite G3's. But the difference between 8.5.1 and 8.6 was perhaps the annoying contol bar at the bottom of the screen, or the ability to use iApps.
But for simple things like WordProcessing and Low qual. Photo editing(and I mean LOW) it's one of Apple's better OS's.