Given my experience with the game up to the middle of Inferno difficulty, I'd have to say that I agree with GameSpy's Dan Stapleton when he makes the assertion that "Diablo III isn't an action RPG" in this new opinion piece. Gone are optimal character builds and the ability to assign attributes - now your character's strength is entirely based on items:
Diablo and Diablo 2 (along with most of the action-RPG genre) have very linear stories, so they possess only one of these two qualities: a character who develops according to the player's choices. As you level up you're given the option of which skills to unlock or enhance, and which to bypass. Diablo allows you to unlock skills by deciding where to place your attribute points each time you level up, and Diablo 2 adds a skill tree to the equation, giving you even more choices of how to develop your character. That's still enough roleplaying to earn it the RPG label, though it's shared with the equally important "action" moniker.
Diablo 3, however, does none of this. As we play, we do earn experience points and level up to unlock new abilities, but we're given no choice in the matter -- reaching a new level simply opens up a new equippable option when we reach a predetermined point in the level progression, much like earning a new gun in a shooter. The result is that my level 55 Monk is identical to every other level 55 Monk on the planet. Sure, I can select which powers I want equipped, but that's no different from a shooter that lets you pick what guns to put in your hands from an inventory. If your character can do everything my character can do, then they are by definition the same character. This Monk doesn't belong to me.
Everything that makes my character unique can be transferred to yours in a trade window. That leaves only gear to distinguish a Diablo 3 character -- gear that is either randomly doled out by drops or purchased on the Auction House, and everything that makes my character unique can be transferred to yours in a trade window. So the only real choice is which gear to keep and equip and which to sell or throw into the wood chipper for crafting materials. I'd argue that gear customization does not an RPG make; there needs to be more than that vestigial tail -- and the sheer habit of referring to all things Diablo as action RPGs -- to justify calling it such.