1. Enthusiastic ignorance
2. Intolerance
3. Indifference
4. Religion (pretty much a blend of 1 and 2)
5. Abuse of power
Enthusiastic ignorance believes we can keep on producing people without exhausting the world's ability to support them all.
Intolerance, especially of the "acted upon" variety we are seeing by ISIS in ethnic cleansing and choices between accepting Allah or losing your head, has the potential to bathe the world in blood.
Indifference to the ignorant and the intolerant allows them to flourish and grow.
Religion looks to a higher power to solve problems and grant boons in worldly affairs that are the problems of mankind's shortcomings as a species. Sorry, but the idea that the group cracking the small end of the egg and believing god's representative on earth must come from the bloodline of the profit Mohamad or from those who won't eat pork, or those who will only eat pork, or dance with rattlesnakes, or any other capricious belief system, will win the ear of god and invoke overwhelming power, kind of doesn't seem to have much going for it in the way of plausibility. Religion siphons off humans into ritualistic activity that rarely solves problems, except maybe the overpopulation problem mentioned at the beginning.
Abuse of power exists at the macro and micro level, micro when a police officer decides he's empowered to shoot an unarmed black man, macro when a bunch of pompous gasbags decide nations should be invaded because theya re friends with someone who badly needs that country's oil, gas, boron, uranium, water or manufacturing facilities.
Are we going back to the barricades again any time soon? I need to buy a Lenin hat circa 1919 if we are.