Allow multiple category selection in listing and datasource webparts
When using a repeater webpart for instance you can only select 1 category. Same is true if you use a documents datasource or a hierarchical viewer webpart. The API allows you to pass in multiple categories when doing a document or object query but we do not have this same ability to do so with any of the out of the box controls.
I come across this use case in roughly every other site build. Unfortunately, it isn't available so we end up performing a no-so-fantastic work around by using the API to get what is needed vs. using OOTB tools.