there isn't anything wrong with the game itself, really.
As far as games go it's basically standard. Some bugs, and some balancing issues...welcome to multiplayer RPGs.
the only flaw the game really has, is gear differentiation and how there isn't any. It's way too easy to gear up. you don't need a 2H staff anymore as a wizard, you can use a fucking broadsword if it has higher DPS. You can even use a bow. There's very few class specific items and most aren't worth bothering to get.
Armour is the same.
It sounded like a good idea on paper but what it did is made gearing incredibly boring and unrewarding. The AH made obtaining better gear too easy. Using an actual currency system is also a hindrance to shit.
Basically, they took Diablo 2 and added a whole lot of bad features that don't work.
Sure they changed a lot for the better, like how you can easily go to most any part of the story specifically, or the companion system. That stuff is good. The skill system took a giant hit for creativity and depth, but it ain't a bad system either.
They'd basically have to remove the AH to really improve anything.