Big problems are a) Adoption (selling the system to SO many independent shops. If you can get in with big REIT companies then that's easy, but 90% of those have smart cards anyway b) Outside of the the super high rent areas (New York, San Fran, etc) most people that use laundry mats are lower income, who may not have smart phones and/or credit cards.
Contrast that to the per-use fee structure, in which case you don't lose any money when you go down to the laundry room and see the machines are broken - you haven't put your money in the machine yet.
It can be that someone frequently over-reserves wash periods. Here's a picture of a more complicated solution involving physical locks http://askersundsflickan.blogg.se/images/2011/dsc05064_13323... . You can only reserve a time with a lock, and you only have one lock.
I have a friend who moved to Switzerland. He says that in his building your laundry times are pre-booked for the entire year. Don't miss your wash day!
An app for managing access to community/shared resources? Meeting rooms, ice rinks, laundry, coop cars, ??