I'm interested in hearing your definition hedwards, if you can flesh it out a little more.
I don't have a particularly good one, it's just that by the time you relax the generally accepted definitions far enough for Diablo to qualify, a good chunk of non-RPGs are suddenly RPGs.
Personally, I'd be more willing to accept Borderlands as an RPG than Diablo.
Personally, I wouldn't consider it to be role playing unless you get to create and customize the character and develop the character throughout the game. Diablo provides very little in that respect. You choose a template and then the actual customization is pretty thin. You can allocate the stat points, choose equipment and spells and that's about it. In practice you tend to choose the same spells as some of them are useful and some aren't. The quests even in Diablo II are relatively fixed leaving very little in the way of guiding the character to alternate endings.
That being said, often, but not always, an RPG will have mutliple endings depending upon how one played it, in cases where that's true you're probably dealing with an RPG.
It does get a bit murky because dungeon crawlers like Diablo and Rogue really ought to be in their own category. Same goes for games like King's quest which belong in the adventure category typically.