There's a lot wrong with Diablo 3. For reasons that should be obvious, but seem to be invisible to Activision/Blizzard and many other publishers, always-online restrictions are terrible for games and gaming, and have had a genuine negative impact on a huge number of Diablo 3 players due to lag or the complete inability to play. The plot seems to take considerable effort to be non-sensical, overwrought drivel when it could and should be a simpler, more focused affair that enhances the world and atmosphere. The real money auction house appears to be a cynical cash-in, unwanted by most (and took an embarrassingly long time to go live, despite being used as justification for the always-online requirements). As many have noted, the early balance is poor, with the initial stages of the game far too easy: Act 1 on normal is far, far too long for such a trivially easy slice of game, and not until a second run through the story on Nightmare - some 10-15 hours in - does the challenge reach about the right level. And did I mention that being essentially an MMO technology-wise, there no support for mods?
This game represents or demonstrates a lot of worrying or just plain disappointing patterns currently afflicting mainstream publisher funded gaming, to the point that, had I not be lucky enough to get my copy free, I probably would have given it a miss. I certainly wont be buying any Activision or Blizzard titles in future on principal - although few of them interest me anyway.
On top of all of this, the actual act of playing the game itself can only really be described as grind: grind to level up and gain new abilities, and grind to find better loot to equip your character with. These grinds even have sub grinds: the grind to collect gold, the grind to level up your blacksmith and jewelcrafters (and the grind to level up jewels with your jewelcrafter), the grind for crafting materials, the grind to equip your NPC companions. In all instances you make slow but steady and inexorable progress up a hierarchy of ever more powerful steps or levels via a repeated, standard activity: bashing monsters and collecting loot. Grind, grind, grindy grind.
Oh, yes, and I've spent well over 100 hours with Diablo 3 and adore it. You see, the thing is, while the criticisms in my first paragraph still stand and still bother me greatly, the fact is that I actually quite like grind.