@happy - Damn woman....your enthusiasm with Diablo III, has me wanting to get it. I barely have time to play any DD as it stands, but.......New! Shiny! *grabby hands* The reality is, I only buy about two games a year: DD:DA was one, so I'm thinking Diablo may be my number 2 buy. Though, to really enjoy it, I think I should wait till time is not so thin.
@OdanUrr - I think the answer to your question would vary among even those of us that play regularly. So I can only answer for myself. The story line is rather simple, but there are interesting elements. Truth be told though, I don't play this game so much for story or even role playing (though I do a little bit, more as a comical farce.....screenshots help with that. Thank you Shepenwepet for that bit of crack! LOL). What I do love and what keeps my playing are the following:
1) Character Creator. It's bloody fun. So much variety, I just can't get bored with it.
2) Builds. Between Discipline points and XP, leveling up the vocations, min/maxing if you choose, and running with various augments, there is just so much variety. And since you get to set this up not only for your PC, but also your Pawn, it gives you lots of opportunities to try new things.
3) Pawns. The AI can be a bit of a joke at times (but I just find that funny....lol. Bad, BAD Shinon for jumping off that cliff to your doom....and for no apparent reason. XD ), but if they are built well, they can do a lot of damage. And they definitely are good for a laugh. Quite often.

The fact that you can share with with other gamers, makes the system even more appealing.
4) Loot. I love loot, and this game has a ton of it. The clothing, armor, weapons and crafting material is extensive. Love being able to mix and match armor/clothing. Finding and upgrading new weaponry and armor, is also great. You really CAN see how improved the items are, when you start fighting enemies with upgraded weapons.
5) Screenshots. It doesn't work as well as taking screenies on your PC, but just the fact that they have this option, is fantastic. Just look through this thread for some of the outrageous photos people have been able to capture. They are a hoot!
Those are just my main reasons for enjoying this game. I can run around upgrading, fighting, looking for photo ops, garnering new loot, and not worry about how invested in the story I am (mostly because I have run multiple play throughs, so I am more than familiar). This makes it a great little diversion type game. And it means I can play in short bursts, and still have some satisfaction.