If you're looking to help some of your own site members get their sites set up, you'll also want to look for a host that answers your questions well. The precise definition of "well" varies for each person: some just want the answer, some just want the task taken care of, some want to be taught how to troubleshoot a problem themselves, some want a mixture of all three. Figure out what kind of answers you look for from your service provider, and check out their support forums (if available). Then ask them some questions of your own. See if they'll be the kind of resource you'll be able to use easily and well.
When I first decided to become a reseller (gah...nearly six years ago), I looked for a good, solid support staff that talked with and to its clients rather than at or down to them. I found good, geeky folks...and about a year later, the company began offering dedicated servers. I eventually moved to their dedicated servers, and while I could conceivably get servers for less money each month with any number of other companies or datacenters, I feel very comfortable knowing that these particular people are taking care of the network infrastructure, that they'll teach me when I have questions, and that they're going to be here for a while. I feel comfortable not just on my behalf, but on my clients'. Since you're asking about what features you should look for, I'm going to make a leap and say that you'll have plenty of other questions as time goes by. You'll want to do your own research, of course (best way to learn); but it's always good when the folks taking care of your servers can tell you their experiences with their particular setup, and guide you through that ground.
In terms of features? H'm...that depends on what you think your customers will need or want, and how you plan to allocate resources. Since I've seen more and more solid, stable applications available that use the LAMP stack (Linux, Apache, MySQL, PHP) I'd offer that. More sites, even small ones, are running some kind of blogging or CMS software, so I'd offer at least one *SQL database with any size hosting plan. I would look for a host that offers an easy-to-use control panel, so that your customers can easily handle some of their own tasks without feeling completely lost.
Again, if I were in the market today, I'd go with a web host that offered a wide range of options for myself and my customers. I'd look for a host that offered, or was thinking of offering, both reseller plans and dedicated servers - so that when I made that move, the transition would be relatively painless. I would look for a host that seems to have plans about where it's going to be in three years, and how it will react if disaster hits its primary datacenter. (It's one thing to handle the day-to-day issues well - it's another to have plans for growth and emergencies, and not all hosts know what they'll do in situations outside the day-to-day.)
Remember, as a reseller, you're not just shopping for your own needs - but those of your customers. It puts an extra spin in things.
Good luck! The comparison shopping can drive you right out of your tree. Still, once you find a home, you may be lucky enough to find a long-term home.
