The original Diablo was great when it came out but by modern standards it suffers from an acute form of reverse sequelitis.
That is, the sequel, Diablo II, took the basic gameplay formula that made Diablo so addicting then added bells and whistles that made it impossible to revert back to the original.
The gameplay here is still mostly solid but the existence of a better sequel makes it pointless to ever play the original Diablo again.