1) Such a society may have difficulty organizing in the face of an external threat.
Iraq faced a significant external threat in the form of a vastly militarily superior enemy with virtually unlimited resources.Was government-organized defense more effective or was spontaneously-organized resistance more effective at preventing this external enemy from achieving its goals?
The Iraqi resistance is anything but disorganized. They have weapon-smuggling channels from other Arab states, they Al Queda and other religious organisations providing training and men. In fact, they have a pretty significant command-and-control structure, it's just that it's very branched and asymmetric.
Just realized that that is totally tl;dr (I get carried away sometimes). But I should point out that this debate is speculative, and will likely remain speculative. If AnCap was a good way to (un)govern a state, one would think that it would have caught on somewhere in the marketplace of sovereign states. If Medieval Iceland is the only example of AnCap--and even that is subject to dispute--it doesn't seem to bode well. Hell, Milton Friedman and his Chicago boys had an opportunity to create an extremely libertarian state that would be enforced by a dictator, and even that ended up quite a bit of statism involved (e.g., Chile's version of Social Security is privatized, but it is very tightly regulated).
This and your previous post I thank you for 
was a good read!