A crappy registrar spams the hell out of you and makes the renewal process long, painful, and full of irrelevant marketing. A good registrar provides you with

A crappy registrar spams the hell out of you and makes the renewal process long, painful, and full of irrelevant marketing. A good registrar provides you with a service that is easy to use, unobtrusive, and provides flexible tools for managing (updating WHOIS, transferring, configuring) your domains. The actual task of domain registration is technically straight forward, but it is the type of service that touches a lot of businesses, so it attracts a the type of business owner who looks to leverage their large customer base. This allows them to push the cost of registration down to the point of being a loss leader. The real value for them isn't in selling registrations, but building a large list of customers to whom they can sell "value add" services.