I can't wait to see how many downmods stupid sheep heap upon me for this:
My big problem with big-L libertarians is that they generally don't believe that markets can fail. Even though we've seen time and time again how supposedly "free" markets can fail - and badly - due to external factors, network effects, or information asymmetry within the market.
But don't tell it to a Randroid, lassiez-faire "capitalists". They're unable to even concieve of the idea that the same aspirin that relieves their headaches in low doses, could destroy their kidneys if they take way too much.