One big thing that springs to mind is the market. What with seeding and mixing of crafted IOs and attuned IOs, they've made it work for a smaller pop game that we have; without any of that, we'd all be running around hoarding SOs or paying two billion for a single LotG or something else ludicrous. If you have even one min/max kind of build on just one toon, you can thank that market work for that. Even back on Live it was becoming unsustainable.
Opening up the Incarnate stuff was also very helpful. Back during Live, near the end, I was really seeing the population gravitating to doing little other than Incarnate grinding, it was seriously killing the game for me.
For me, it's removing the gate-keeping around costume parts and the asymmetrical thing for gloves, boots and shoulders now.
I suspect as time goes by, the updating of the infrastructure behind the scenes is going to be more important as well.