At least some of the popup windows would benefit from an application filter. Ideally, it would be that type-and-click kind of filter you are already using in some places. The particular use case is picking items for graphs, that was pointed out by anettu on IRC, and that I find very beneficial.

It might be even more powerful, if one were allowed to select All for hosts and/or groups and could only filter by application.The filter should also work for adding item prototypes.