although i believe that Diablo is a great game ( both Diablo 1 and 2 ) i don't know if i would put them in the RPG category... in Pure RPG's there is at least a bit of interactivity with other people, you talk with others etc. Diablo is simple Hack-n'-Slash
Not that this is a bad thing... after all, Hack n'Slash is the only element of RPG games that ComputerRPGs do better than Pen & paper RPGs. They are not as deep as real RPGs ( up until now at least - i can't know what will come next )