That we can see our clients' accounts in real time, that they are updated instantly as soon as there's an update to them- even if it's just by one character! We've been using this software since 2014 when we started out with my first client.
There were things I liked about it then, but still many problems too which needed fixing (like lack of customisable fields!). We solved those issues during 2015 and now use almost all the features available including auto SMS reminders whenever client status changes or bill payment date approaches. It has saved us from manually entering invoices into Sage 50 at least 20 times now.