I'll bite. I should note before I get going here (and get called a libtard or retardican) that I am only critiquing libertarianism. I fully am aware that other system have their flaws, and there are no easy, obvious answers to any of these problems. I am not suggesting that any other system is "the answer" to any of the issues below:
In my heart I am a libertarian, maybe even an anarchocapitalist to a point (let the government build roads and keep the military basically). I love the idea of self-governance and responsibility, and the idea that whatever we work for we get to keep and use as we see fit, and generally agree with Penn when he waxes philosophical... but as I look around the world I just don't see it working in practice.
The way I see it, in the real world (some amount of) regulation is a necessity because as the scales slowly start to tip in favor of those with money/resources, and money is viewed as the only thing that matters, that snowball grows of control. I mean hell, even with regulation you have tons of horrible stuff happening in the name of saving money like industrial disasters, known fatal car issues, etc... One need only look back at recent history to see that the rich have absolutely no problem taking advantage of the poor to whatever extent they can get away with, for as long as they can get away with it. You generally have to force them to do the right thing.
Crony Capitalism (and worker exploitation) is the inevitable end point without regulation. They arent aberrations or break downs of a system, they are built in and unavoidable. I'd even go so far as to say there is no such thing as Crony Capitalism, its just a word assigned to part of the system that we dont like, but it isnt some other form of capitalism.
I also think libertarianism only works on the "frictionless plane", which is to say conditions need to be perfect. For example, everyone needs to be equally and correctly informed to make the correct decision, and its implied that everyone has equal opportunity to thrive and succeed.
The last point I'll make for now, because this became more rambling than I thought, is that of Charity and heath care. Penn seems much more confident that I am that if only we gave rich people tax breaks they would magically start using all of that money to provide for the poor. Nope, if its one thing rich people have shown us, its that there is never, ever enough money. Even multi billionaires who could never spend their fortunes thirst for more money. Some people eventually do reach this conclusion, but they generally have to achieve levels of wealth an income that are almost beyond comprehension... Which they likely wouldn't have acheived if they had paid their workers better or charged a lower price for their product (thus raising the amount of money in the pockets of everyone else). But they never would think of doing that, which is why you have to regulate stuff like a minimum wage.
I also don't think health care should be a for-profit system. There is no reason that CEO's of health care, insurance, and pharma corps should be making tens of millions of dollars. Yes, make a good living and live well even... but to live obscenely lavishly on the backs of peoples health who can barely (or not at all) afford it is, in my opinion, a disgraceful part of the system.
tl:dr - Libertarianism is awesome in theory, but breaks down in practice, and the worst parts of libertarianism are worse than the worst parts of other systems.