Nice-to-haves:
API
auto-broadcast upcoming open spots to Twitter/Facebook on a schedule (say 2 days before)
native iOS apps or at least responsive websites (latter is a must-have)
integrate directly with Square
indirectly would be OK
integrate with FreshBooks (or whatever invoicing thing you use)
crazy idea: price set by a computer, automatically adjusting price based on a combination of increased demand (percentage of future blocks filled) + percentage who show up (paying customers) + satisfaction (thumbs up)
set a minimum (what you think you're worth and what you need to make a living) and maximum (what you need to make a really nice living, without gouging customers or too negatively effecting demand)