What actually matters in a registrar long term?

I'm running domains for multiple clients now, and I'm starting to realize that what sounds good in the beginning isn't always what matters later on.

What have you learned to value in a registrar after working with them for a while?
What makes day-to-day management smoother (or painful) once volume grows.

Good communication, not too overpriced compared to godaddy and namecheap, sometimes offers really help and I think customer support is pretty important.

being able to bend the rules at times and being flexible

Customer Service. Especially since your success with your clients is reliant on that.

Pricing should not be your main focus, but with sufficient large portfolio, usually you will also get better pricing with most registrars.