Thucy-
"What is the defining idea that unites every leftist? That's what I'd really like to learn in this thread."
Fuck, I don't know if there is a defining idea which unites every leftist. For me, the reason I feel more comfortable on the left is because I am familiar with poverty. It seems to me that a person mired in poverty, which according to the capitalist model of distribution, is a systemic and necessary condition of the economy, really doesn't have the ability to defend his or her other rights that I hold we as a society have the means to guarantee to all. Under the capitalist model, who you are is completely a function of how many resources you control. If you're poor, you have access to substandard education. You have access to substandard legal representation. You have access to substandard transportation and health care. It seems to me to be unjust to write off huge swaths of society from these basic rights, but that's the system we have developed.
It also seems foolish as well as unjust. First, because it squanders a ton of talent. How many scientific breakthroughs have we not developed because we refuse to make the effort to make everyone scientifically literate? How many great works of art have gone uncreated because the artist has been too busy working to try and make a living wage and too tired at the end of the day because of it? Second, because inequal distribution of wealth tends to really fuck up a society. The more that economic power is concentrated within a society, the more important such power becomes. Which means that those who have it can privilege themselves and their outlooks to a greater extent than they could if wealth were more broadly diffused. This tends to create a feedback loop, and the end of the loop involves violent revolution. Third, accepting the premise that more economic activity is better for society as a whole, society gets a lot more bang for its buck on marginal dollars controlled by people in the bottom half of the economic distribution graph than in the top. We as a society get more out of a dollar in the hands of someone who made 20,000 USD last year than we do someone who made 1,000,000. So, ensuring broader distribution of wealth also tends to create more wealth.