The central error of the economic policies of the mainstream American "left" seems to be a failure to consider how others will react to those policies and how those reactions will undermine the goals of the policy. Let me explain this.
Example #1: The minimum wage.¶
Equality is more important than fairness, so we should force employers (on the theory that they're wealthier than their employees which is isn't necessarily true itself, especially when you count debt) to pay their employees more than their labor is worth to the employer.
Not only does this harm the employer, it probably won't result in the worker being paid more. If I'm an employer and I want to hire someone for $6 an hour - and the potential employee thinks this is worth it for them - but the government won't let me employ them for less than $10 an hour, I probably just won't hire that person. So the result isn't low-skill workers being paid more but low-skill workers being unable to find jobs. And hence of course more justification for "the poor are downtrodden in this capitalist economy, we need the government to help them!" Obviously if the minimum wage is below the value of the job to the employer but above the worker's best option elsewhere, it can result in the worker getting more money, but it's at best an unproven assumption that that's the case more often than not.
I should also point out that this concept seems inherently untenable because it can't be upheld consistently. What about a favor? Are you allowed to do someone a favor for $0 an hour? Or if there's some special case against 0, how about $3? What's the actual difference? What about children in schools, who work several hours a day and aren't paid shit?
Example #2: Anti-discrimination.¶
In a world with a completely free market economy, being prejudiced would be a horrible business strategy. It deprives you of potential employees and customers and vastly harms your reputation. But leftists saw a problem where it never existed and passed The Equal Pay Act. Notice what that means. Paying women less than men for the same work has literally been illegal in the US for 50 years. Keep that in mind when liberals claim women still face that systematically - it means either they're wrong or government intervention hasn't solved the problem.
You can't stop people from being prejudiced; but you can force them to hide their prejudice so they don't pay the natural costs of it as described above, to be prejudiced in more subtle ways and make up other excuses for their asymmetry, which, if anyone is convinced which certainly people are according to the leftist narrative, will only improve the reputation of prejudiced people.
But more sinisterly, discrimination being illegal actually discourages employers from hiring women or minorities because if they do they're at risk of being accused of paying them less and sued for it, and claims about such discrimination are often widely open to interpretation - note that the law uses language like "under similar working conditions". Who do you think's going to define what's similar?
Example #3: Gun control.¶
The only people using guns to commit crimes are the people who aren't going to follow your laws against gun ownership; any of them that don't already have guns are probably willing and able to obtain them illegally and will be pleased to know their law-abiding victims won't be armed as well.
I read a wise sentence once: If guns are outlawed, only outlaws will have guns.
As for the argument that other weapons such as tasers are sufficient for self-defense and guns only make it easier to cause more harm than necessary (even the legality of pepper spray and taser ownership is flaky): that may be true, but it's worth considering that there's a cultural element of conservatives to own guns for reasons like symbolic support for the constitution, which doesn't fully carry over to other weapons, so such measures would still reduce the likelihood of armed victims. (Also, given the nature of the current criminal "justice" system, it's probably better for everyone to kill the attacker than get them arrested, even if it isn't required for your own safety.)
Of course, the biggest reason to oppose gun control is that it is a euphemism for "gun monopolization". Destroying all guns in the world would be one argument, but preventing civilians from having them when the most powerful criminal organization in the world has them either way is impossible to respectably advocate. But that's beside the point here.
Example #4: Communism.¶
Most statist leftists aren't full communists, but favor moving closer to it. Equality is an alluring goal, but the less people are allowed to keep what they produce, the less motivated they'll be to produce. You can increase equality, but besides being theft it's going to drastically reduce the total wealth available; freeloading more and more becomes increasingly viable.