You need to ask about uptime.
http://uptime.besthostratings.com/webhosts-uptime.php
I have found a couple of hosts that say 99.9% uptime, but they actually had 98.9% on their stats.
But that's pretty much all you can ask them unless you plan to get a dedicated server. If you're going with a dedicated server you can ask about processor type/speed, and RAM. See, in most hosting plans you're in a shared environment. Multiple people with multiple websites on the same server - it's pointless to ask about the processor on the server because you won't get to use all of it anyway.