All the fundamentals
Your clients need secure, performant, and findable websites. They need to be able to grow their websites over time, without getting stuck with workflows that feel slapped together and hacked. You need to build for them with components and services that are priced predictably so you can protect your own margins while helping your clients invest cost-effectively. And, if and when your clients seem unable to deliver the content you need to release (or the content they need to succeed), you need to be ready to offer content development resources to augment their capacity (and yours).
These are the fundamental requirements shared by all promising website teams and owners. But web agencies might need a little more...