Build must be able to do at least MP5 to be considered viable.
MP2-MP3 is a fun build.
MP0, well everything is viable in MP0.
MP6 to MP10 is only for cookie cutter builds. According to the stats for a barbarian, 96% of the high level paragon players use both Ruthless and Weapon Master.
Two passives does not a cookie cutter build make.
I use this build http://us.battle.net/d3/en/calculator/barbarian#acQiVk!dYZ!bZZZZc for MP10 and other high MPs.
Dual wield with high cc and high cd along with the constant 20%cc from overpower and revenge. Used with 8.5% LS and the 9% rend->life from Blood Lust rune, it is quite fun.
The idea is to remain in ground effects with as many mobs as you want, overpower AoE dmg about every 5 secs, revenge AoE and heal almost on global, bash +24% dmg and spamming the balls out of rend. Echoing fury fear has a bonus here, as you are constantly fear bombing and rending the circulation of mobs.
It's funny to get frozen and take a massive hit to your hp, stood in all sorts of ground effects, dozens of mobs beating on you and actually see your hp shoot back up to full.
It also allows me to do things like...solo MP8 skele king from 90% to dead then go and resurrect the dead WW barbs before starting on the butterfly woman.
And Zoltan Kulles slow time and teleport shenanigans have little effect with rend.
Ignore pain would ideally be replaced with battle rage +30% dmg, but it's a nice oh sht for times it doesn't quite go your way as well as being a god send for succubi and occultists.
For lower Mp's IP is replaced with battle rage and Blood Lust rune for rend is replaced with ravage for range.