A part of the problem may be our expectations for the economy. Since this isn't an mmo with lots of post game content, once the hard core get their upgrades there is less of a market to sell stuff to. Buying a new piece of gear makes everything less than that gear worthless for that player. With nothing to bring new blood into the game the market is bound to saturate. Still plenty playing but you have to be more patient nowadays.
I hate that this game is kind of feeling like a Trojan horse to wow. I vowed never to play an mmo again but the post game of Diablo is leaving me wanting more.
They really need to do an endless dungeon/tower thing with rankings and leader boards and aestetic awards and such. Something to keep the treadmill going longer.
Yes some sort of siege mode/arena/temple run thing with increasing fast/powerful waves of enemies would also really help.