The ability to bring up the Pay Dialog from anywhere in a script.
Currently, it seems that the only way to initiate payment is through the pie-menu when you right-click (unless the object is set to Pay as the left-click default)
So any type of interaction with a user needs to be paused to tell them "okay, now right click on me and pay me". If an item will require user-interaction and/or processing both before and after payment, this is a rather non-user-friendly way to implement it.
Being able to call a function like llPay in mid-script to bring up the Pay Dialog immediately would be much simpler and flow more smoothly.