Lack of Customization, Bad DRM, Subpar Standards
Customization: This is my biggest point of contention with the game, that there is zero character customization. As you progress you unlock skills and your stats automatically increase, exactly as in a game like WoW, but wildly different than the Diablo series (particularly part 2). There is no skill point allocation, no stat choosing, etc. Instead you will play the exact same cookie-cutter character as everyone else. By the time you are a level 50 barbarian you will have the same stats, runes, skills, and even look the exact same as every other level 50 barbarian there is. The only differentiating factor is your gear, but this is a shallow way of making people feel like they've made a unique character. The notion that you get to "pick" your runes/skills is a joke. They are unlocked in a specific order, you always have access to the same skills/runes, there is nothing customizable about it. In other words, your barbarian will be the same as the other 2,000,000 barbarians out there without one tiny lick of difference.
Simplicity: I have not played the game on Inferno difficulty yet, so take that for what you will, but everything else has been ridiculously simplistic. How simplistic? I have yet to die. I'm not kidding or exaggerating, I literally have yet to die in this game and I have played for hours.
DRM: The always-online feature is abhorrent and inexcusable. That people are seriously defending the inability to play a single-player game they paid for because a server is down is mind-boggling. There is no reason for this kind of garbage. Blizzard is treating its customers like enemies and rendering people incapable of playing the game that they bought. And what of those without a reliable and strong internet connection? Left out in the cold, I suppose. Companies like CDProjekt and GOG.com have managed to eliminate DRM entirely from their games and profit wildly. Hell, Diablo 2 has no DRM whatsoever and is easily pirated, yet it's still bought en masse 12 years after its release. A huge negative for the game.
Subpar Standards:
The graphics in the game are barely acceptable. I would say they are akin to the first Torchlight's graphics except with a slightly darker palette. However you also lack the ability to zoom (except while in your inventory, and then only if you select the option from a menu). The details on your character are nonexistant and the camera angle is pretty poor. Given that they managed to have a zoom function in Warcraft 3 while still having the same camera-style I can't even fathom this decision making, except for the fact that the graphics look pretty 2008, at best.
The sound is actually fairly decent, but the voice acting is atrocious. The stilted and wooden voice actors sound like something out of a made-for-tv SyFy movie, and that's being kind. The one exception is the man who voices Deckard Cain, who is a huge cut above the rest of the cast. I have not played every iteration of character/gender but the 5 that I have tried all sounded abysmal (with the absolute low point being a female monk).
From what I have read numerous features were cut from the final game and it really shows. The crafting feels pathetically shoe-horned and is akin to the most dumbed-down MMO crafting you can imagine. The "runes" you get to differentiate your skills were apparently supposed to originally be loot you'd find, but that went away as well. There's no PvP out of the box, there's no LAN, and there's no more than 4 players to a game. Ultimately this all feels like a rush-job, which sounds absurd given how long this game has been in development, but it seems that is precisely what happened.
Ultimately, the terrible design decisions, the lack of any character customization whatsoever, the subpar standards, the rushed feeling, and the godawful DRM hold this game back from being anything other than straight-up mediocre. I could not be more disappointed.