First, to maintain backwards-compatibility, traditional top-level domains are blacklisted. For example, you can't register "com" on Handshake because it would conflict with Versign-managed .coms.

Second, the top 100,000 alexa domains are reserved, so you can't register "Google" on Handshake and redirect it to your site selling pet rocks (this was a big problem with previous attempts at decentralized systems). Site owners of reserved Handshake names can claim their names here.