The problem is anarchy dissolves. Even the purest-minded anarchists forming a tiny commune somewhere have to agree to rules that govern them and their interactions, or else they quickly fall apart. At the very least, they'd have to have basic unspoken rules such as "Don't punch people in the face for no reason." You could probably create a tiny website with no overt "rules" if you only let your close friends on, people who shared your philosophy and goals for the site, but even then, you've established rules in the sense of setting up borders and denying non-members access in addition to the unspoken agreement to treat each other in mutually beneficial ways.
Once you allow membership to grow, you have to have some sort of moderation or your site collapses under the Asshole Wave. All it takes is one Asshole who disregards the unspoken assumption that people won't get punched in the face for no reason and your once idyllic rec room is full of a handful of frightened individuals clutching couch cushions in front of their faces and cowering in separate corners, unwilling to risk interaction for fear of face-punching.
In other words, when there are no rules, when there is pure anarchy, then the strong can impose their will on the weak and there is nothing to stop them. (Unless the weak band together and overthrow the strong, which is war and/or rebellion, and afterward quickly returns to the strongman scenario unless the weak take steps to establish a government that protects individual freedoms.) This is what strident Internet Anarchists and Internet Libertarians (i.e. 14-year-olds who espouse these philosophies on the internet) fail to understand, which is that one of the fundamental roles of government is keeping the peace. It is possible to have a strong and supportive government that isn't OMG REPRESSION; not all laws and restrictions are limitations. A good law is one that supports personal liberty while preventing the strong from imposing on the weak simply because they can.
