A mediocre dungeon crawler, not made by the key people from the real Diablo team.
Blizzard owns IP so they slapped the Diablo label on it - it was going to sell no matter what. As consumers we don't care enough to be informed before we purchase.
I completely flip-flopped over this game - the few hours of play I was disappointed, then for about a week I had a blast (barbarian), then massive disappointment set in and never went away.
Act 1 is ok. Act 2 is getting lame. Act 3 has epic action but feels Confined, hurried, and just empty (consider this: how much Time transpires in the story during all of Act 3. Three hours??). And Act 4??? Don't get me started - the biggest rush-job in Blizzards history and they know it.
Loot: Ruined, utterly ruined, primarily this was due to BOTH auction houses (not just RMAH), and the crafting system was utterly ineffective in eliminating excess items from the game. The game was farmed 50 times as much due to the RMAH, which even further polluted the virtual economy with items. The belief was players wanted loot "falling from the sky" constantly. In reality, this made everything worth far less - to the gameplay, to the player, and in terms of actual value.
Skills: By no means did I find this system more interesting or engaging than the previous titles.
Major oversight: Awesome physics and sense of power to the barbarians attacks, true for the monk as well. The other three classes - What were they thinking?! If you try a barb or monk first then the other classes -you are screwed, you will hate the feel of them. If a barb can smash a table with his ax and it flies everywhere, why on earth would a powerful wizard be unable to move the table more than a foot with a meteor from the sky??? So stupid and somehow nobody notices. Instead of correcting the Gameplay and Feel, they go on and on about the online requirement and absence of enchantments.
All enemies are the exact same to fight against, for the entire game.
The environment changes every 30 seconds (overkill! .. and makes every area seem way too small), yet they all play the exact same.
The choice of game difficulty, and the implementation of it, was completely wrong from absolutely every angle you can slice it. Even the decision to introduce a "4th difficulty" created a host of problems across the board, I won't even get into it here.
And finally, rather than increasing the depth of the game (sales were guaranteed), thus No Need to "cater to new players", they DECREASED the depth of the game. FAIL!!