[sokoloff]

There are several other sources of barriers. Network effects is one; goodwill/brand is another; execution/operational excellence, etc.

Think about Ebay, Paypal, Facebook, Amazon. The vast majority of what they do is pretty far from rocket science. (By that, I mean a startup could create a technically credible competitor for their core business.) However, those companies have erected enormous barriers to entry in the form of network effects, brand/trust [of buyers], network effects, and brand/breadth/operational excellence.