Ask HN: What makes a great domain name registrar?

Following the recent zoho.com downtime, an interesting discussion emerged about choosing a registrar. I always assumed registrars were "just" middlemen between you and the registry (ICANN for example), and that the cheaper the better.

Apparently I'm dead wrong. A bunch of people said that MarkMonitor was the gold standard, and those guys don't even have a pricing page so I assume their service must be a lot more expensive than your average registrar.

Hence my question: what makes a great registrar? What job do they really do? Why would one be willing to pay a lot more for what may seem like the exact same service (forwarding an entry and a fee to a registry)?