The ability to run a reliable hosting platform. It's actually more about how you put together your servers; simply because servers that are poorly constructed will cost a lot more to support over time. In particular, as your server use grows and you get more and more accounts, your performance will suffer if you don't detect and ban malicious activity. So, getting your servers hardened is actually important - and often not considered or thought about much by newcomers, for understandable reasons.
Here are a couple of examples. There's a spammer trick where they just try every possible email address they can think of under a domain until they get a match. This puts a huge load on the server as you'd imagine, and banning it brings things back to normal quickly. As another example, scripts (Joomla, Wordpress etc) get hacked regularly. If you are able to prevent the rate of that with something like mod_security and CSF, you can reduce your support levels and increase your reliability substantially. Another thing you may not have thought about is limiting outgoing emails per hour by default, which will catch many spammers, and preventing outgoing email direct on port 25, which will catch the remaining wannabe spammers.
Providing good help documents is also useful - it can allow many people to solve their issues without needing to come to you. Every time you get two or three people asking the same thing, write up a short knowledge-base article on it. You'll find this will make it faster to serve people, and you'll learn to write and document in the process.
Just something to think about in getting going; hope this helps.