I think balancing an RPG for the end game can be pretty difficult because you have to account a lot of theoretical party complexity. On one hand, you don't want encounters to be too hard, you want the pacing to match the development of the player's party. On the other hand, players can be very good at exploiting mechanics. Although SoA doesn't have this as much, Throne of Baal has many, many moments where my group could just right click entire enemy groups to death without breaking a sweat, seldom having to actually use any of my upper tier abilities to clear most enemies. And really, despite its rough spots, I was able to clear Watcher's Keep before clearing the Underdark in my second playthrough (granted, I chose to skip fighting the final boss, knew I stood no chance) ... and that place is pretty freakin' ridiculous. Even some of the fiercest challenges can be beaten with the right party makeup and tenacity.
I know Obsidian has talked about its current stance on difficulty, adding a few modes to enhance difficulty, and to tailor enhanced levels of difficulty by adding diversity and size to monster packs instead of just buffing damage and health.
But I was wondering, perhaps, if there should, or could be a rudimentary little check after a certain point, to see how "easy" the last fight was. On higher difficulty levels, I was thinking, what if there were something like L4D's ai director? Towards the end game, this director would kick in after checking to see whether you're right-clicking things to death or not. If you are, it would further add to enemy diversity and perhaps unlock certain abilities for monsters, to further enhance difficulty? And if you start wiping a lot afterwards, it would turn back the difficulty to normal.
Just an idea anyway. Not saying it should be all game long, or exist on the easier difficulties. Just that, in spite of a developer's best intentions, a game may still be "too easy" for some players, given the right items and party make-up.