I don't know what art is but I know what I like.
I don't know what an RPG is but I know what I like. I like D2. It has been debated in many forums whether or not D2 is a true RPG. I don't care. Many RPGs have been modeled after D2 one way or another.
Now one of the major draws of D2 is the ability to customize your character based on skill and attribute selection. And there are very distinct character types. Several games which are referred to as RPGs have this type of setup even though they might be lacking in the quest department. That is fine with me because I like to hack and slash with the ability to customize my character more than I like playing the role of step 'n fetch it.
So for me an action oriented game with distinct customizable character types is an RPG. And if it isn't an RPG according to some folks because it doesn't have a strong story line with quests and multiparty dialogue that can spin the yarn in 7 different ways I don't care as long as I have fun.
I like DD. I like the hack and slash in it. I wish the character trees and skill systems were more improved, but it is a great game in spite of some quests such as the ones in Stormfist castle.

The question is not, Can they reason? nor, Can they talk? but, Can they suffer?
~Jeremy Bentham