Top 10 "must have" features for any web hosting company (in no particular order):
Reliability. As mentioned above, downtime is a no-no. It will happen, but if you can't guarantee 99.99% uptime, it isn't worth a glance. Every minute the network is down, I lose money - not just my investment, but potential profit.
24/7 phone/chat support. If I have a billing issue, I should be able to correct it whatever my schedule is. If I have to work my hours around my host, my business will be better served elsewhere. If I'm suddenly locked out of the system, the server crashes, or any other unforeseen scenario plays out, I want to know that my host is on top of the issue mere minutes after it happens (if that).
Ease-of-use and fluidity are a must. If I have to dig for information or spend hours on the phone with support, then it isn't worth it.
PHP and MySQL versions should always be in the latest series range. No one uses PHP 4 anymore, and if you offer it, your company is a bust. Would you enjoy going to your favorite restaurant and having them tell you they don't know how to make the special you've been waiting for? I think not! The number of allowed sub-domains, scripting language support, databases, OS platform, etc is all important features.
Look around at some of the top-tier companies and try to undercut their prices. You can't reasonably expect to charge as much as Go Daddy does, even if you provide similar resources - you're just not Go Daddy - you can't match their level of support or service. Being too proud means being naive.
If you leave your customers feeling sour, you can bet "word of mouth" will spread across social media and other sites. Make sure you leave a smile on your customer's face every time you do business with them. If word gets out that you're a jackass, you'll be treated like one by potential clients - or, you may just not be treated like anything, by anyone.
Space / Bandwidth. Need I say more?
What other features do you offer? CDN? Malware Scanner? Mobile optimization? More perks and goodies generate interest.
Initial setup, security/emergency services (i.e. reboots), monitoring services, etc are all additional must-haves.