(Yes Detmer linked the last one, but its widely read because Blizz responded to it publicly)
These are excellent threads describing the D3 problems, why D2 worked, and why D3 doesn't.
Mainly, the problem is, D3 has a brick wall you bang your head against in Inferno, D2 doesn't - that's the key difference. In D2, the hardest difficulty was easy enough no matter how badly you screwed up your build and stats, you were still a _viable_ character no matter what (though you would clear inefficiently).
Through experimentation people got weird builds working in D2. Javazons, Melee Sorcs, etc. It was fun. In D3, you are pigeonholed into a few select skills to even progress. That's not fun.
In D2, you mowed through crowds of monsters (Cows anyone? Chaos Sanctuary?) in the highest difficulties. You feel powerful and awesome. In D3, you ran in circles from FOUR champion monsters until they died for 5 minutes. You feel weak.
And here's my list of reasons why D3 failed:
- Quests are not rewarding, do not offer unique rewards. In d2, they offered you a high level rune/gem/socket, skill point, etc
- Game types. In D2, you could name the game, trade game, rushing game, foothills farm 2 game, achievements game, etc.
- Lack of chat channels to form a community around - we have to resort to external forums. You can't even display your avatar to the world like in D2.
- NV buff - my friends don't want to leave games to help
- Boring loot, items are bland, they removed over 50 interesting affixes that gave +skills, +skill procs, +chance to cast X skill, +faster block, +faster recovery, legendary/set-only affixes... They "streamlined" the gear so there is no longer any differentiation between caster gear and melee gear.
- Less item affixes that are unique, stuff to proc skills, immunity, faster cast, block speed, block recovery, etc
- Legendaries are crap, not usable past maybe 5 levels beyond their ilvl
- Legendaries contain too many random affixes
- Less gem types, less legendaries
- Clunky UI, I have to tab though several pages to find the skill I want
- Inferno isn't hard, but its CHEAP, its full of 1-2 shots, that makes it incredibly frustrating, it does not make me feel powerful. Consider the Jar of Souls event (Act 1 Crypts), that was fun, that massive amount of skeletons made you feel powerful.
- IAS nerf is overdone, especially with a RMAH value on items
- First kills beyond normal don't drop rares - people used to create new characters just to farm good Runes/Rares on certain quests (Hephaesto, etc)
- Nerfing MF on chests and pots, removing treasure goblins, chests, etc farming areas, is removing playstyle choices
- Crazy unkillable combinations of traits on champion packs too often, and usually too close to each other
- Normalized everything by weapon damage means there is little gear differentiation between caster loot and melee loot
- Enrage timer is an arbitrary restriction on playstyle
- Cheesy storyline
- The lack of a level cap (lvl 99 was so far away the level cap was as good as non-existant in D2) meant there is a goal of leveling up towards the next skill point, you always felt like you are doing something even if lousy loot dropped.
- Game philosophy of forcing players to get 5 NV then kill boss (and nerfing every other method) is stupid
- Lack of character customization options compared to D2, and I don't mean dye colours and gear style. Here, I mean stat allocation, Skill points allocation, Playstyle choices, Lesser gem colours...
- Only a few builds are usable in Acts 3/4 because of gear dependency
- RMAH has reduced the sense of achievement of finding rares, shifted item hunting towards gold hunting ("find item to sell to buy item")
- Maps in the open world are a lot less random (same perimeter all the time, the exits are always in the same corner)
- No Ethreal items, barely any superior items, no transmuting. They even made items just be 0 durability when it reaches 0. In D2, they broke and disappeared.
- Only act 1 felt the right mood, the rest of the game had little horror atmosphere, bright colors everywhere
- Dialogue is so cheesy and the player (dialogue) is so haughty. I'm the one broken out of immersion by lines my character wouldn't have said. Act 3 is beyond stupid.
- Low replayability due to free respeccing, lack of character attachment
- Lack of character uniqueness/identity/ownership - all builds are freely changeable, many builds in the late game are similar or identical - no character permanence + lack of customizability
- And there isn't even PvP or PKing.
- Botting still exists, a bot costs about $30 and you can make that investment back in a day. The current rate of gold is about $2.5 per mil gold (though in the first weeks it was over $50). You can still bot over 400k gold per hour in Nightmare.
- The ability to dupe gold using the gold AH (by cancelling your auctions by setting your system clock back so D3 thought you have 5 minutes to cancel your item - this caused any existing bidder's gold to go to you, and had a good chance the bidder's gold also returned to him - it sometimes does/doesn't). This exploit crashed the economy (inflation) really early in June and the price of gold plunged from $30+ to $12 per million within 3 days until it got fixed.
Game design and balance is fluffed. I don't know what Jay Wilson was thinking.