My biggest problem with Diablo 3 is this:
This is a graph I made including the general power level of stats / items / enchantments.
The X axis is level 10 - 60 (started at 10 for various reasons and added 60.5 and 60.75 for act 3 and 4 of Inferno).
The Y axis is the stat compilation (I have not added monster data though, but I assume they rise in stats even more)
As you can see the growth is incredibly exponential.
This means that progressing through the late game is incredibly hard and that it's very hard to balance.
It also screams to me that they want people to use the AH / RMHA because everything that should be powerful at your level can only be found where you should be at 2 levels in the future (making a comparison of where/when each tier drop / equip level)
Also, this means that any expansions will probably be even harder to balance than what we have now.