The UI is currently a major weakness in the engine simply because it is under-documented and difficult to understand. The source code does provide some examples on the use of Slate, but it incorporates overly complex use cases that do not apply to most projects.
What is needed for Slate is the following:
A complete reference document. The current documentation is insufficient and provides incomplete functionality examples.
Video tutorials demonstrating the creation of Slate projects from start to finish: both Slate Applications using Unreal Viewports and the use of Slate within a viewport. Something of the order of the Blueprint Tutorial video series would go a long way towards understanding Slate.
Interfaces with Blueprints.
A slightly adjusted SlateViewer that removes some of the asserts when testing some of the widgets. Also, a bare-bones SlateViewer-type app with only the main functionality: menu, buttons, treeview, image slider control, and of course a viewport.
For your consideration.
EDIT: I forgot to add that a Slate Visual Editor would also be very important