What features would you expect from a high-end reseller plan (cPanel/Fantastico)? 99.9% or higher uptime, quality support and private nameservers are a given, but what else?

High resource usage (Ram. CPU).

Most important thing they should be reliable. Check there whereabouts before paying them. You can see many examples in this forum where hosts disappear in hours.

also one that doesnt offer too many features for a small price

I would say a listed phone number (or a management e-mail address) that you can use to contact management. In other words not a sales only phone number or address. No one wants to have problems or issues with their host but if you do there needs to be a way to walk up the ladder until you can get issues resolved.

I agree with BrilliantHosts, Solid Support is essential in the reseller hosting market. You can read the board and find several horror stories regarding Support not being reachable. In my own opinion, a company who only provides a SALES hotline is a company that is only concerned about one thing. SALES. Not that it's a bad thing to be concentrated on from a Hosting Companys standpoint, however, Customers expect the same level of SUPPORT, if not better, than they did when they initially received SALES support. When you do only SALES over the phone, it should show how dedicated the company really is when it comes to support.

How about the software offered? cPanel/Fantastico a given and I'd say a professional sitebuilder, but what else?

My minimum requirements - RV Skin, Fantastico, Dedicated IP for your hosted accounts, If possible private name server with your own dedicated IP. WHM/Cpanel, Urchin

I've checked out RVskin several times over the years, but I somehow don't get what's so special about it? What do I miss?