I have never played a Diablo game before, this either makes me the best or worst person to review this.
Never Fear though the game is actually pretty awesome but for reasons that mankind will probably never understand. I as much as anyone understand the compulsion to level up and complete every side activity despite how little you actually gain from it. Diablo and I assume other games in this genre are completely about that exact fact. Much like the rise in free to play games having you grind out meaningless levels, Diablo hooks much into the same addiction but without you paying to avoid not leveling for hours. In fact like the best games of this nature (Dark Souls most recently comes to mind) you get hooked on shear progress and find excuses to avoid finding a stopping point. Frequently you will think, "Well I'll just stop when I get to the next level" then after you do you go "Well I'm nearly done with this dungen..." and its a vicious cycle.
It's the game itself that is the most fascinating because there is so little yet so much to it. You at any given time have 10 things and decisions on your mind regarding the current scenario, but at the same time are completely mindless. I would say roughly 50% of the time playing the game I couldn't tell you what dungen I'm in or why or even if I'm looking at the screen. The graphics while sharp and artistically beautiful just become shapes that in the corner of you looking at the map and your power levels. This blows my mind because it's so fun and engaging but still requires only about 10% of your attention. I have to wonder during development of games like these if they even know if it will be fun until they release it.
Alright let's get the formalities out of the way.
There is a girl and an asteroid crashes into her house hurting he grandpa or something, and that turns out to be an Angel warning of Diablo coming or whatever... Honestly if you can follow or get through the story without skipping most of the dialog I applaud you. It's insane nonsense that may or may not be good I honestly couldn't tell you.
As you would expect with a Blizzard game the graphics are technically solid. It never goes too crazy with newer lighting effects or high poly counts but makes up for it in fantastic art design. Everything has a very clean and pleasant feel with all the environments having an awesome amount of background detail. The game also never leaves 60fps (or stays very close) so no matter how many effects and characters are on screen it plays very smooth.
If you are a fan of games like Borderlands, Dark Souls, World of Warcraft, etc... you will undoubtedly enjoy this. I wouldn't say anything revolutionary is happening here and the story is a throw away despite the great CGI cinematics that you get every act change. Have a stack NPR to listen to? Diablo 3 is a great way to do so. (... I have never listened to NPR)