It wasn't until I started playing Path of Exile that I realized everything I didn't like about Diablo 3. The biggest problem is the auction house and gold as currency. Diablo 2 was amazing because there was no true currency, the game was about finding awesome items and trading them for other awesome items. It fostered community and gave the game a great feel, as Path of Exile does now. Diablo 3 "feels" empty and cold. You farm gold to buy items on auction house to farm more gold to buy more items.
The other issue is class customization. Diablo 3 offers almost no real choices in character customization. You get to max level, you can swap around stuff at will but in the end you're probably only using one or two "viable" builds. One Wizard is no different from the next Wizard, except in gear choices, which are not generally choices but a treadmill of item level. Diablo 2 offered you hundreds of points of customization through stat points and skill points, Path of Exile has almost 1400 different choices.
Path of Exile feels how I wanted Diablo 3 to feel. It is much more of a true sequel to Diablo 2 than the current state of Diablo 3. I'm really really really hoping that the Diablo 3 devs will look at what GGG has accomplished with just a small team and learn from it. It's not too late to save Diablo 3 with an expansion that removes the Auction House and adds a real ingame community.