1. Not just Good, but Excellent Support. Both polite, knowledgeable and clear in their explanations. Able to understand client queries and knowing the right questions to ask if they don't. Always there, always fast to respond.
2. Great system administrators that understand the optimal server setup like they know their name. That goes for both setup, optimization and security.
3. A fully-featured solution - you have to have the essentials of your project at one place for easier management. Tools for web building, security certificates, SSH access etc. are used by almost everyone nowadays so don't make your clients look for them elsewhere.
4. Personal treatment - each client comes with their own personal projects that has its own requirements and means to achieve their goals. Those have to be addressed with the needed personal care and understanding. Knowing just that the "customer 325425 is using WP" just won't cut it.
5. Looking for the future - this corresponds with another important thing and that is being unique. In this large pool of hosts you have to know your business very well and always keep an eye on latest technology so you can always strive to get better and provide a better environment to your clients.
Just the first 5 that came to mind, there are certainly some others holding a different weight.