Licensing Requirements: In many cases licensing requirements can be an almost absolute barrier, at least for a time. If a local government only licenses one firm to sell electricity locally it is essentially impossible for other firms to compete in that market. On the other hand, such barriers are still just costs. Consider local cable TV. Most markets only allow a single provider, but various companies selling home satellite systems have been taking market share from local cable companies.
Patents: As Kodak® learned when it tried to enter the instant camera market a few years back, a patent can be an insurmountable barrier to entry, even for a large corporation. Companies can, if profits are high enough, attempt to find alternative processes that produce similar goods or services without violating existing patents, though this is usually very costly, and may not succeed.