The paper-thin character progression. By that I mean power progression, stuff that actually makes you grow in power.
Every time I think about coming back, I'm reminded that no matter how much I farm and no matter how much gold I have, I won't become any more powerful than I was soon after hitting lvl 20- which is a very short-lived milestone in itself.
I'm actually quite curious as to what people do with all their gold. The game seems to have reached a point where 99.99% of players know all the best farms and basically want for nothing and so there is nothing to stimulate trade. In fact, the only thing you ever see on trade are bots selling Z-keys. I wouldn't mind this so much if there was a goal I could set that would make some tangible difference for my toons. But there is nothing like this beyond some title track skills. I just can't find the motivation to play it anymore because there is nothing meaningful to aim for.
I find it really hard to maintain enthusiasm for farming when the only thing I have to gain from it is the act of farming itself, as the rewards are more or less meaningless. It just really saddens me that despite all the great content in this game, the progression ceiling will always remain very shallow. It's both funny and sad when you think about how there is basically no power differential between a fresh 20 and a lvl 20 with 7k hours dedicated to it. One just "looks" better.