Native support for non-metric units in the GUI.
Current "expression system" works, but has two problems. One is simply that almost every numerical entry requires clicking on the fx button and then closing the expression in a multi-step process.
The other problem is that every constraint in a sketch shows up in the dependency graph as an expression, often greatly expanding each element. Makes it much more difficult to get useful information from the dependency graph.
This may be inherent to the basic libraries used in FreeCAD, but I have never encountered another multi-unit-system program that requires the expression style entry. Typically one just sets the defaults in preferences and never needs to think about it again.