An option to create rules (similar to the change visibility feature) to determine whether an action will run or not.
Notice this dropped quietly over the weekend - absolutely brilliant! Allows for a step-change in the complexity of our apps :)
This would be critical for me in my app - if a check box in a form is selected, then a notification would be sent, if it isn't then it wouldn't - as an example. I think a lot of people would want this too.