Act 4 is my least favorite, as its hard to relate to, but it has its moments, like killing/freeing trapped souls. That doesn't get old.
Hanging out at the fireplace with Tyrael - an angel (and possibly archangel) - and Cain (last of an ancient order serving him) is pushed to the surreal. He's part of this Act's home base, and is practically playing the role of the innkeeper. At least there's no town drunk, after all, this is the legendary Pandemonium Fortress, last outpost of Heaven's forces before you reach Hell.
Speaking of Heaven and Hell, why does this story so aggressively pussy-foot around the obvious - where's God in all this? Maybe there is no head guy, and its an oligarchy, like in Hell? It's absence is deafening in its silence. Of course the real answer is that Blizzard isn't looking to do anything crazy controversial here, they just want to borrow some popular culture as background for their hack and slash game. Like the way they mixed Mayan, Thai, and Indian cultures to make up Kurast. But then why is the hierarchy so aggressively Christian then, why not throw in all kinds of gods?
Back to leveling, I want to reach 30 and get my Frozen Orb cannon.
Slogging my way through hell, and I realize something while fighting some Abyss Knights. If I just hit them with lightning bolts it takes forever to kill them. If I get up close and freeze them with glacial spikes and static field and/or lightning them, it takes a while to kill them. Either way, its expensive and it takes time. Or I could just drop two blizzards on them, and done. By trying to save mana and use cheaper spells, it costs more mana and time. Just go right for the big guns.