@hostEONS hit the nail on the head. Network uptime and reliability would be #1, along with that I'd determine what type of DDoS protection is offered as those two really go hand in hand.
Secondly you'll want 24/7/365 support for those just incase moments. Always research the company, check reviews, how long they've been in business etc.
Little tricks such as can you find an address and phone number on their website are helpful as well.