The perspective from a hardcore gamer, someone who has been expecting this game for ten years (actually more like seven, because that's when 1.10 came out), as someone who has played the game intensely enough to realize that when you enter the Halls of Vaught, you toggle Walk back on to not have your armor rating reduced to zero, as someone who played mods extensively to increase the longevity of the game because of the playstyle, this game is a massive let down.
The formula in Diablo III is the same as Diablo II's: and that's pouring enemy water into a player sink and seeing what goes down the drain. For most this game is enough to enjoy it. It poses enough of a challenge to not be pointless drivel. The randomness of elite packs and items is also psychologically addicting. Casuals don't care about the mechanics of skills or how they apply, or the finer details of itemization versus skill selection.
In honesty, for casuals, the game ends at Hell Diablo. If you are okay with not playing through Inferno and the irritation it brings, if you don't have a completionist complex like I do, you should be fine, and it is a relatively enjoyable experience. It is not the best among MMOs, but that sixty you're paying now means access to the game for a very long time. Blizzard, at least, is very good about patches, which is one of the only things that keeps games like this relevant for years on end.
There is no story. If you have played Starcraft, you have actually played Diablo III. Frankly, I'm surprised at how much Metzen managed to screw up the world that other authors have managed to create with the novels. It's all predictable, but fortunately, I don't care about the plot.
The server issues have calmed down immensely since release. That is a plus.
However, the nature of the items forces you to engage in the use of the auction house. Which is down half of the time. Normally this would be perfectly fine for an MMO, but only if all five classes are not competing for the same exact items. I hope you have someone to twink you items, because I'm broke.
On a side note, Diablo III really reminds me of Nexon's Dragon Nest, as far as its item system goes. The nice thing about DN is that it's free. but also lol nexon