We recently helped a client improve their conversion rate 36% by adding a guest checkout as the first step in their checkout process. Just as in a grocery store, customers want the fastest checkout possible. Guest checkout removes both a step and the perceived mental hurdle of account creation.
We suspect, but have not tested, that guest checkout subsequently improves repeat purchases. By circumventing passwords, the anxiety of trying to remember a password is removed.
This doesn't mean we should abandon registrations entirely. We discovered in customer surveys that some users, especially older ones, feel as though their order will be "lost" if they're unable to associate an account with it.