The RPG Vault has posted a nice article by Blizzard North founder and Diablo creator Max Schaefer, in which he discusses what makes an RPG, an ARPG, and where Diablo fit into that metric.
I was fortunate enough to work on all the games in the Diablo series, a title that is today most commonly considered an action RPG. The arguments about how to classify it were strangely heated, with some people insisting that it was a full-fledged RPG, and others who refused to accept that it was an action RPG… or even that such a category was valid.
Our goal in making Diablo was to bring the rich world of RPGs to audiences that were less stat-obsessed and intense, and that craved more visceral entertainment. We wanted to de-nerdify RPGs. To us, what was great about the genre, what made the D&D sessions we all had when we were kids so fun, was the vivid image that you’d be in a dungeon, face to face with an evil animated skeleton king, armed with a kick-*** mace, ready for battle.