I was trying different builds in nightmare based on builds I found here and I stumbled across this one that really seems to be working.
I've only died once so far to a pack of arcane and shielding rare creeps but I killed them on return. I handled Leoric easy and I am ready to move on. I thought I would share my build and the thought process behind it.
The Build: http://us.battle.net/d3/en/calculator/barbarian#WSXViP!adc!ZZaaZZ
First off this is a build meant NOT to spend much fury. It's a build based on getting to max fury and staying there.
Frenzy w/Triumph: The attack speed is boss and works really well with this, also the healing per kill will work wonders in large groups. However in boss battles feel free to to swap it out for Bash with the Stun rune.
Furious Charge w/Relentless Assault: I use this to charge in and around large groups as it will reset the cool down and deal a lot of damage while keeping you mobile, as well as it's a good opener into rares or Unique monsters.
Leap w/Iron Impact: If I don't charge in I'll leap in... if I do charge in I'll sometimes leap right away adding damage and adding to my armor it can also be great for getting out of bad situations.
Revenge w/Vengeance is mine: This is a must for ANY barbarian in ANY build.. if you do not have this then you do not know what you are doing.. go stand in the corner until you understand! Also.. when I get the rune that increases it's chance to proc I'll prolly take that instead.
Ignore Pain w/Iron Hide: This is obvious for bosses, rare monsters and large packs. This will allow you to stand down most of the major problems you come across. When I get ignorant is Bliss I may sub that for Iron Hide....
Wrath of the Berserker w/insanity: Okay I don't have insanity yet but when I get it it'll be what I use a 100% damage increase will help you mow down even the toughest of foes.
Pound of Flesh: A must to help find health globes and get a stronger return from them... still might NOT be needed.. I'm going to play around with this some.
Bloodthirst: The life steal of this really can help in the long run, even more so in long fights but again this one can prolly be swapped around to taste.
Berserker Rage: The core of this build. +25% damage when at full furry.
I have been mowing down minions and rares alike with this build with only one death so far. When you combo in Wrath of the Berserker + Berserker Rage + Fully Stacked Frenzy. You are a killing machine. Plus the healing of Revenge and ignore pain and added health globes give you the survival to stand in there and fight toe to toe. Charge and Leap give you the ability to get in and out of groups and fights quickly as needed. It might feel a little strange to not want to spend fury but it is working for me so far. A nice touch is so far I have a 1k+ crit on this build without using Wrath.
Last notes: I have tried this 2-handed and duel wielding. IT works just as well on one as the other. So you can use whatever the hell you want with this build.
Also when working with a group of friends or public game.. you could probably switch Ignore pain with War Cry and the rune of your choice to make it a little more party friendly.