The ease of use, its ability to be used across multiple platforms (mobile/desktop), it's flexibility in being able to adapt as needs change over time without having to rewrite code every few months. I dislike how slow the system can become when you have upwards of 50 users accessing this calendar at any given point during business hours. If there are more than 25 people using an event room simultaneously then performance will suffer significantly. We're solving our need to house all employee scheduling information into one place while also allowing access from mobile devices so we don't lose employees who forget their phone by accident.