I'm not a WoW player so skipping that part altogether really.
Diablo 3 however failed to appeal to me for a few rather simple reasons
Diablo 2, for me, was all about customizing. I could make myself a mage with an absurd amount of HP, I could build a necromancer who flooded the field with minions. I could go druid and either destroy you with spells, let my animal companions rip you to shreds, or go beast mode on your face and pretend I'm melee. Disablo 3 lacked this customization You were left with a bunch of basic skills you had little to no choice in, set combos, set ungh.. It was like they stripped down the complexities of the classes, you couldn't play them your way anymore, you had to play them the way blizzard wanted you to play, and for me that was a major killer for the game.
Diablo 2 the story intrigued me slightly, not much, but slightly. I didn't expect certain parts of the 'story' (i use the term loosely) to unfold as they did. Diablo 3 was predictable. <SPOILERS> The moment it was revealed that girl was special, I instantly went "companions gonna go demon" the moment that kid showed up in Act 2 my mind went "Demon" it just wasn't the same.
I don't think I died while playing this game. I mean, I came close once or twice thanks to critical hits etc, but nothing was able to kill me outright. Diablo is meant to kill you. I remember throwing myself at Andariel several times before I got the kill, I remember Diablo absolutely destroying me. In Diablo 3 it just wasn't hard.
I actually stopped playing before it went live. But the idea of a real money auction house just didn't do it for me. I don't really need to go any further into this
I remember playing Diablo 2 over the course of a week or so getting to the end of the game. (Faster once I'd learnt where everything was for additional playthroughs) I remember playing diablo 3 for a day and beating it. This goes hand in hand with the difficulty before, I understand that Diablo 2 probably wouldn't have taken nearly as long had I not died so much. But still rather annoying personally.
Diablo 3 is not a Diablo game, it's blizzard trying to cash in on nostalgia. Sad to say loads of us bought in before we knew what we were getting, which is probably what paved the way for their new dungeon keeper bs.