Please correct me if I am wrong. But one thing that monks have that may be unique to monks (but again, I may be wrong on this) are a combination of skills that can (1) have massive damage buffs
They're damage buffs for a few seconds. Barbarians have damage buffs that are lasting.
Battle Rage: Increases all damage by 15% AND Critical Hit Chance by 3% for 120 seconds. All the runes for it are awesome.
Marauder's Rage - Increase damage bonus to 30%
Ferocity - keep the damage buff forever
Swords to Ploughshares - in conjunction with pickup radius, you can keep a steady stream of incoming healing. Also synergizes with Witch Doctors and Demon Hunters
Into the Fray - This one rune, which already buffs all damage by 15%, further buffs damage through the use of Wrath of the Berserker, because Into the Fray generates more fury to keep it active.
(2) our ability to debuff monster defences (MoC/Overawe for 48%, EP/TFIW for 12%).
Barbarians can debuff too. Threatening Shout. Reduces all damage done by monsters by 20%. We can further reduce their damage by using Falter. There's another synergy rune (Grim Harvest) for Battle Rage: Swords to Ploughshares which increases chance to drop health globes by 15%
Sure, Monks can reduce enemy defenses, which in turn leads to more effective DPS, however Barbarians can straight up increase DPS and keep it active for a very long time. It overshadows everything Monks have. Why would I want to run FiTL, Overawe and TFIW when I can simply run Wrath of the Berserker and destroy everything in one fell swoop?
And with both of those, we can come up with some combination of that AND a strong spirit spending attack (like WoL @ 1200% weapon damage of SSS/SA for ~2300% single target weapon damage).
OR, go into Wrath of the Berserker mode, whip out my Hammer of the Ancients which ALWAYS crits, thanks to Battle Rage: Into the Fray and the innate increased chance to crit depending on how much fury I have. It destroys everything and regenerates a massive amount of life thx to LS.
I still have no idea why people when arguing our passives are weak, choose to use barb's as a base for their argument.
Because we are also a melee class. The comparisons I made were both offensive and defensive related passives. They also touched on the fact that because Barb skills were more generic (ie: damage buffs are straight up +dmg, more crit, or more crit dmg) they are usable at all levels of gearing, not just low or top end. I didn't just compare barbs either, I also used Demon Hunters as an example of that same concept.
There is a reason why people play Barbs, and a reason why they are cheaper to gear and plvl with than Monks. If Monks were so awesome and effective while also being cheap to gear for than everybody would play a Monk.