Smart system engineering.
On some of our more processor-intensive program features like the De-duper or Newsletter Manager, we've created background processes that do most of the work off-line. This balances the load on the server and lets you go on with your day while your server works for you.
Good customer service.
You'd think this would go without saying, but it's really hard to find these days. Many web tech companies structure their support so that you spend most of your time with a customer service rep who really doesn't know anything about the servers they are supporting or the sites on them. We offer our servces at a slight premium so that we end up with a limited number of customers who are well taken care of.
We take a "hacker's eye view" when building applications or maintaining our servers. This means using a firewall and locking down unnecessary ports. It also means running a limited number of applications on a server and patching them frequently.